Merge RFC4 (#1865)

This commit is the result of the squashing of rfc4_dev branch in a single
commit. It implements mostly RFC 4 related work.

* Grid handling:
  - remove obsolete and presumably unfinished implementation of grid catalog functionality
  - all grid functionality is in grids.cpp/.hpp
  - vertical and horizontal grid shift: rework to no longer load whole grid into memory
  - remove hgrids and vgrids member from PJ structure, and store them in hgridshift/vgridshift/deformation structures
  - build systems: add optional libtiff dependency. Must be explicitly disabled if not desired
  - add support for horizontal and vertical grids in GeoTIFF, if libtiff is available
  - add GenericShiftGridSet and GenericShiftGrid classes, relying on TIFF grids, that can be used for generic purpose grid-based adjustment
  - add a +proj=xyzgridshift method to perform geocentric translation by grid. Used for French NTF to RGF93 transformation using gr3df97a.tif grid
  - deformation: add support for +grids= for GeoTIFF grids
  - horizontal grid shift: fix failures on points slightly outside a subgrid (fixes #209)

* File management:
  - add a filemanager.cpp/.hpp to deal with file related work
  - test for legacy proj_api.h fileapi
  - proj.h: add proj_context_set_fileapi() and proj_context_set_sqlite3_vfs_name() (fixes #866)
  - add capability to read resource files from the user writable directory

* Network access:
  - build systems: add optional curl dependency
  - add a curl-based default implementation for network related functionality
  - proj.h: add C API to control network functionality, and optionaly provide network callbacks
  - add data/proj.ini with default settings
  - add a SQLite3 local cache of downloaded chunks
  - add proj_is_download_needed() and proj_download_file()

* Use Win32 Unicode APIs and expect all strings to be UTF-8 (fixes #1765)
  For backward compatibility, if PROJ_LIB content is found to be not UTF-8 or
  pointing to a non existing directory, then an attempt at interpretating it
  in the ANSI page encoding is done.
  proj_context_set_search_paths() now assumes strings to be in UTF-8, and
  functions returning paths will also return values in UTF-8.

1 files changed, 267 insertions, 0 deletions

diff --git a/docs/source/specifications/projjson.rst b/docs/source/specifications/projjson.rst
new file mode 100644
index 00000000..6bcce585
--- /dev/null
+++ b/docs/source/specifications/projjson.rst
@@ -0,0 +1,267 @@
+.. _projjson:
+
+================================================================================
+PROJJSON
+================================================================================
+
+PROJJSON is a JSON encoding of
+`WKT2:2019 / ISO-19162:2019 <http://docs.opengeospatial.org/is/18-010r7/18-010r7.html>`_,
+which itself implements the model of
+`OGC Topic 2: Referencing by coordinates <http://docs.opengeospatial.org/as/18-005r4/18-005r4.html>`_.
+Apart from the difference of encodings, the semantics is intended to be exactly
+the same as WKT2:2019.
+
+PROJJSON is available as input and output of PROJ since PROJ 6.2.
+
+The current version is 0.2.
+
+Schema
+------
+
+A JSON schema of its grammar is available at
+https://proj.org/schemas/v0.2/projjson.schema.json
+
+History
+-------
+
+* v0.2: addition of geoid_model in VerticalCRS object.
+* v0.1: initial version for PROJ 6.2
+
+Content
+-------
+
+The high level objects are:
+
+* Coordinate Reference Systems (CRS):
+
+  - Common ones:
+
+    + ``GeographicCRS``
+    + ``GeodeticCRS``
+    + ``ProjectedCRS``
+    + ``CompoundCRS``
+    + ``BoundCRS``
+
+  - More esoteric ones:
+
+    + ``VerticalCRS``
+    + ``EngineeringCRS``
+    + ``TemporalCRS``
+    + ``ParametricCRS``
+    + ``DerivedGeographicCRS``
+    + ``DerivedGeodeticCRS``
+    + ``DerivedProjectedCRS``
+    + ``DerivedVerticalCRS``
+    + ``DerivedEngineeringCRS``
+    + ``DerivedTemporalCRS``
+    + ``DerivedParametricCRS``
+
+* Coordinate operations:
+
+  - ``Transformation``
+  - ``Conversion``
+  - ``ConcatenatedOperation``
+
+* Others:
+
+  - ``PrimeMeridian``
+  - ``Ellipsoid``
+  - ``Datum``
+  - ``DatumEnsemble``
+
+Examples
+--------
+
+GeographicCRS
++++++++++++++
+
+The following invokation
+
+::
+
+    projinfo EPSG:4326 -o PROJJSON -q
+
+will output:
+
+.. code-block:: json
+
+    {
+        "$schema": "https://proj.org/schemas/v0.1/projjson.schema.json",
+        "type": "GeographicCRS",
+        "name": "WGS 84",
+        "datum": {
+            "type": "GeodeticReferenceFrame",
+            "name": "World Geodetic System 1984",
+            "ellipsoid": {
+                "name": "WGS 84",
+                "semi_major_axis": 6378137,
+                "inverse_flattening": 298.257223563
+            }
+        },
+        "coordinate_system": {
+            "subtype": "ellipsoidal",
+            "axis": [
+            {
+                "name": "Geodetic latitude",
+                "abbreviation": "Lat",
+                "direction": "north",
+                "unit": "degree"
+            },
+            {
+                "name": "Geodetic longitude",
+                "abbreviation": "Lon",
+                "direction": "east",
+                "unit": "degree"
+            }
+            ]
+        },
+        "area": "World",
+        "bbox": {
+            "south_latitude": -90,
+            "west_longitude": -180,
+            "north_latitude": 90,
+            "east_longitude": 180
+        },
+        "id": {
+            "authority": "EPSG",
+            "code": 4326
+        }
+    }
+
+
+ProjectedCRS
+++++++++++++
+
+The following invokation
+
+::
+
+    projinfo EPSG:32631 -o PROJJSON -q
+
+will output:
+
+.. code-block:: json
+
+    {
+        "$schema": "https://proj.org/schemas/v0.1/projjson.schema.json",
+        "type": "ProjectedCRS",
+        "name": "WGS 84 / UTM zone 31N",
+        "base_crs": {
+            "name": "WGS 84",
+            "datum": {
+                "type": "GeodeticReferenceFrame",
+                "name": "World Geodetic System 1984",
+                "ellipsoid": {
+                    "name": "WGS 84",
+                    "semi_major_axis": 6378137,
+                    "inverse_flattening": 298.257223563
+                }
+            },
+            "coordinate_system": {
+                "subtype": "ellipsoidal",
+                "axis": [
+                {
+                    "name": "Geodetic latitude",
+                    "abbreviation": "Lat",
+                    "direction": "north",
+                    "unit": "degree"
+                },
+                {
+                    "name": "Geodetic longitude",
+                    "abbreviation": "Lon",
+                    "direction": "east",
+                    "unit": "degree"
+                }
+                ]
+            },
+            "id": {
+                "authority": "EPSG",
+                "code": 4326
+            }
+        },
+        "conversion": {
+            "name": "UTM zone 31N",
+            "method": {
+                "name": "Transverse Mercator",
+                "id": {
+                    "authority": "EPSG",
+                    "code": 9807
+                }
+            },
+            "parameters": [
+            {
+                "name": "Latitude of natural origin",
+                "value": 0,
+                "unit": "degree",
+                "id": {
+                "authority": "EPSG",
+                "code": 8801
+                }
+            },
+            {
+                "name": "Longitude of natural origin",
+                "value": 3,
+                "unit": "degree",
+                "id": {
+                "authority": "EPSG",
+                "code": 8802
+                }
+            },
+            {
+                "name": "Scale factor at natural origin",
+                "value": 0.9996,
+                "unit": "unity",
+                "id": {
+                "authority": "EPSG",
+                "code": 8805
+                }
+            },
+            {
+                "name": "False easting",
+                "value": 500000,
+                "unit": "metre",
+                "id": {
+                "authority": "EPSG",
+                "code": 8806
+                }
+            },
+            {
+                "name": "False northing",
+                "value": 0,
+                "unit": "metre",
+                "id": {
+                "authority": "EPSG",
+                "code": 8807
+                }
+            }
+            ]
+        },
+        "coordinate_system": {
+            "subtype": "Cartesian",
+            "axis": [
+            {
+                "name": "Easting",
+                "abbreviation": "E",
+                "direction": "east",
+                "unit": "metre"
+            },
+            {
+                "name": "Northing",
+                "abbreviation": "N",
+                "direction": "north",
+                "unit": "metre"
+            }
+            ]
+        },
+        "area": "World - N hemisphere - 0°E to 6°E - by country",
+        "bbox": {
+            "south_latitude": 0,
+            "west_longitude": 0,
+            "north_latitude": 84,
+            "east_longitude": 6
+        },
+        "id": {
+            "authority": "EPSG",
+            "code": 32631
+        }
+    }