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, 91 insertions, 0 deletions

diff --git a/docs/source/operations/transformations/xyzgridshift.rst b/docs/source/operations/transformations/xyzgridshift.rst
new file mode 100644
index 00000000..30537032
--- /dev/null
+++ b/docs/source/operations/transformations/xyzgridshift.rst
@@ -0,0 +1,91 @@
+.. _xyzgridshift:
+
+================================================================================
+Geocentric grid shift
+================================================================================
+
+.. versionadded:: 7.0.0
+
+Geocentric translation using a grid shift
+
++-----------------+-------------------------------------------------------------------+
+| **Domain**      | 3D                                                                |
++-----------------+-------------------------------------------------------------------+
+| **Input type**  | Cartesian coordinates                                             |
++-----------------+-------------------------------------------------------------------+
+| **Output type** | Cartesian coordinates                                             |
++-----------------+-------------------------------------------------------------------+
+
+Perform a geocentric translation by bilinear interpolation of dx, dy, dz
+translation values from a grid. The grid is referenced against either the
+2D geographic CRS corresponding to the input (or sometimes output) CRS.
+
+This method is described (in French) in :cite:`NTF_88`
+and as EPSG operation method code 9655 in :cite:`IOGP2018` (§2.4.4.1.1
+France geocentric interpolation).
+
+The translation in the grids are added to the input coordinates in the forward direction,
+and subtracted in the reverse direction.
+By default (if ``grid_ref=input_crs``), in the forward direction, the input coordinates
+are converted to their geographic equivalent to directly read and interpolate from
+the grid. In the reverse direction, an iterative method is used to be able to find
+the grid locations to read.
+If ``grid_ref=output_crs`` is used, then the reverse strategy is applied: iterative
+method in the forward direction, and direct read in the reverse direction.
+
+Example
+-------------------------------------------------------------------------------
+
+NTF to RGF93 transformation using gr3df97a.tif grid
+
+::
+
+    +proj=pipeline
+        +step +proj=push +v_3
+        +step +proj=cart +ellps=clrk80ign
+        +step +proj=xyzgridshift +grids=gr3df97a.tif +grid_ref=output_crs +ellps=GRS80
+        +step +proj=cart +ellps=GRS80 +inv
+        +step +proj=pop +v_3
+
+Parameters
+################################################################################
+
+The ellipsoid parameters should be the ones consistent with ``grid_ref``.
+They are used to perform a geocentric to geographic conversion to find the
+translation parameters.
+
+
+Required
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+.. include:: ../options/ellps.rst
+
+.. option:: +grids=<list>
+
+    Comma-separated list of grids to load. If a grid is prefixed by an ``@`` the
+    grid is considered optional and PROJ will the not complain if the grid is
+    not available.
+
+    Grids are expected to be in GeoTIFF format. If no metadata is provided,
+    the first, second and third samples are assumed to be the geocentric
+    translation along X, Y and Z axis respectively, in metres.
+
+Optional
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+
+.. option:: +grid_ref=input_crs/output_crs
+
+    Specify in which CRS the grid is referenced to. The default value is
+    input_crs. That is the grid is referenced in the geographic CRS corresponding
+    to the input geocentric CRS.
+
+    If output_crs is specified, the grid is referenced in the geographic CRS corresponding
+    to the output geocentric CRS. This is for example the case for the French
+    gr3df97a.tif grid converting from NTF to RGF93, but referenced against RGF93.
+    Thus in the forward direction (NTF->RGF93), an iterative method is used to find
+    the appropriate shift.
+
+.. option:: +multiplier=<value>
+
+    Specify the multiplier to apply to the grid values. Defaults to 1.0
+