Implement RFC 2: Initial integration of "GDAL SRS barn" work

This work mostly consists of:
- a C++ implementation of the ISO-19111:2018 / OGC Topic 2
  "Referencing by coordinates" classes to represent Datums,
  Coordinate systems, CRSs (Coordinate Reference Systems) and
  Coordinate Operations.
- methods to convert between this C++ modeling and WKT1, WKT2
  and PROJ string representations of those objects
- management and query of a SQLite3 database of CRS and Coordinate Operation definition
- a C API binding part of those capabilities

This is all-in-one squashed commit of the work of
https://github.com/OSGeo/proj.4/pull/1040

15 files changed, 113 insertions, 2 deletions

diff --git a/docs/Makefile b/docs/Makefile
index 2540fdff..e6a02fdd 100644
--- a/docs/Makefile
+++ b/docs/Makefile
@@ -51,9 +51,19 @@ help:
 clean:
 	rm -rf $(BUILDDIR)/*
 
+.doxygen_up_to_date: ../Doxyfile ../src/*.cpp ../include/proj/*.hpp
+	../scripts/doxygen.sh
+	touch .doxygen_up_to_date
+
 .PHONY: html
-html:
+html: .doxygen_up_to_date
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	# Undoes the hack done in scripts/doxygen.sh
+	@sed "s/io::Convention_/io::Convention/g" < $(BUILDDIR)/html/development/reference/cpp/io.html | sed "s/>Convention_/>Convention/g" | sed "s/_WKT2/WKT2/g" | sed "s/_WKT1/WKT1/g" > $(BUILDDIR)/html/development/reference/cpp/io.html.tmp
+	@mv $(BUILDDIR)/html/development/reference/cpp/io.html.tmp $(BUILDDIR)/html/development/reference/cpp/io.html
+	# Undoes the hacks of scripts/generate_breathe_friendly_general_doc.py
+	@sed "s/<em class=\"property\">namespace <\/em>//g" < $(BUILDDIR)/html/development/reference/cpp/cpp_general.html > $(BUILDDIR)/html/development/reference/cpp/cpp_general.html.tmp
+	@mv $(BUILDDIR)/html/development/reference/cpp/cpp_general.html.tmp $(BUILDDIR)/html/development/reference/cpp/cpp_general.html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
 
diff --git a/docs/source/conf.py b/docs/source/conf.py
index 434e4d73..612c7c7e 100644
--- a/docs/source/conf.py
+++ b/docs/source/conf.py
@@ -34,6 +34,7 @@ import bibstyle
 extensions = [
     'sphinx.ext.mathjax',
     'sphinxcontrib.bibtex',
+    'breathe',
 ]
 
 # Add any paths that contain templates here, relative to this directory.
@@ -346,3 +347,7 @@ texinfo_documents = [
 
 # If true, do not generate a @detailmenu in the "Top" node's menu.
 #texinfo_no_detailmenu = False
+
+breathe_projects = {
+    "cpp_stuff":"../build/xml/",
+}
diff --git a/docs/source/development/reference/cpp/common.rst b/docs/source/development/reference/cpp/common.rst
new file mode 100644
index 00000000..c1a28d37
--- /dev/null
+++ b/docs/source/development/reference/cpp/common.rst
@@ -0,0 +1,8 @@
+.. _common:
+
+common namespace
+----------------
+
+.. doxygennamespace:: osgeo::proj::common
+   :project: cpp_stuff
+   :members:
diff --git a/docs/source/development/reference/cpp/cpp_general.rst b/docs/source/development/reference/cpp/cpp_general.rst
new file mode 100644
index 00000000..3b47d01e
--- /dev/null
+++ b/docs/source/development/reference/cpp/cpp_general.rst
@@ -0,0 +1,7 @@
+.. _cpp_general:
+
+General documentation
+---------------------
+
+.. doxygenfile:: general_doc.dox.reworked.h
+   :project: cpp_stuff
diff --git a/docs/source/development/reference/cpp/crs.rst b/docs/source/development/reference/cpp/crs.rst
new file mode 100644
index 00000000..2abea378
--- /dev/null
+++ b/docs/source/development/reference/cpp/crs.rst
@@ -0,0 +1,8 @@
+.. _crs:
+
+crs namespace
+-------------
+
+.. doxygennamespace:: osgeo::proj::crs
+   :project: cpp_stuff
+   :members:
diff --git a/docs/source/development/reference/cpp/cs.rst b/docs/source/development/reference/cpp/cs.rst
new file mode 100644
index 00000000..b168213f
--- /dev/null
+++ b/docs/source/development/reference/cpp/cs.rst
@@ -0,0 +1,8 @@
+.. _cs:
+
+cs namespace
+------------
+
+.. doxygennamespace:: osgeo::proj::cs
+   :project: cpp_stuff
+   :members:
diff --git a/docs/source/development/reference/cpp/datum.rst b/docs/source/development/reference/cpp/datum.rst
new file mode 100644
index 00000000..1fee5f8a
--- /dev/null
+++ b/docs/source/development/reference/cpp/datum.rst
@@ -0,0 +1,8 @@
+.. _datum:
+
+datum namespace
+---------------
+
+.. doxygennamespace:: osgeo::proj::datum
+   :project: cpp_stuff
+   :members:
diff --git a/docs/source/development/reference/cpp/index.rst b/docs/source/development/reference/cpp/index.rst
new file mode 100644
index 00000000..bedfbe52
--- /dev/null
+++ b/docs/source/development/reference/cpp/index.rst
@@ -0,0 +1,19 @@
+.. _cpp:
+
+================================================================================
+C++ API
+================================================================================
+
+.. toctree::
+   :maxdepth: 1
+
+   cpp_general
+   common
+   util
+   metadata
+   cs
+   datum
+   crs
+   operation
+   io
+
diff --git a/docs/source/development/reference/cpp/io.rst b/docs/source/development/reference/cpp/io.rst
new file mode 100644
index 00000000..9da0f680
--- /dev/null
+++ b/docs/source/development/reference/cpp/io.rst
@@ -0,0 +1,8 @@
+.. _io:
+
+io namespace
+------------
+
+.. doxygennamespace:: osgeo::proj::io
+   :project: cpp_stuff
+   :members:
diff --git a/docs/source/development/reference/cpp/metadata.rst b/docs/source/development/reference/cpp/metadata.rst
new file mode 100644
index 00000000..b3960122
--- /dev/null
+++ b/docs/source/development/reference/cpp/metadata.rst
@@ -0,0 +1,8 @@
+.. _metadata:
+
+metadata namespace
+------------------
+
+.. doxygennamespace:: osgeo::proj::metadata
+   :project: cpp_stuff
+   :members:
diff --git a/docs/source/development/reference/cpp/operation.rst b/docs/source/development/reference/cpp/operation.rst
new file mode 100644
index 00000000..bc3f201d
--- /dev/null
+++ b/docs/source/development/reference/cpp/operation.rst
@@ -0,0 +1,8 @@
+.. _operation:
+
+operation namespace
+-------------------
+
+.. doxygennamespace:: osgeo::proj::operation
+   :project: cpp_stuff
+   :members:
diff --git a/docs/source/development/reference/cpp/util.rst b/docs/source/development/reference/cpp/util.rst
new file mode 100644
index 00000000..1aba2954
--- /dev/null
+++ b/docs/source/development/reference/cpp/util.rst
@@ -0,0 +1,8 @@
+.. _util:
+
+util namespace
+--------------
+
+.. doxygennamespace:: osgeo::proj::util
+   :project: cpp_stuff
+   :members:
diff --git a/docs/source/development/reference/functions.rst b/docs/source/development/reference/functions.rst
index fb29fd6c..5d27999f 100644
--- a/docs/source/development/reference/functions.rst
+++ b/docs/source/development/reference/functions.rst
@@ -571,3 +571,8 @@ Various
     :type `direction`: PJ_DIRECTION
     :returns: :c:type:`int` 1 if angular output is returned, otherwise 0
 
+C API for ISO-19111 functionality
++++++++++++++++++++++++++++++++++
+
+.. doxygenfile:: proj.h
+   :project: cpp_stuff
diff --git a/docs/source/development/reference/index.rst b/docs/source/development/reference/index.rst
index 9e92384c..1d39b1d0 100644
--- a/docs/source/development/reference/index.rst
+++ b/docs/source/development/reference/index.rst
@@ -9,4 +9,5 @@ Reference
 
    datatypes
    functions
+   cpp/index.rst
    deprecated
diff --git a/docs/source/index.rst b/docs/source/index.rst
index c367cafb..33fd096d 100644
--- a/docs/source/index.rst
+++ b/docs/source/index.rst
@@ -4,7 +4,7 @@ PROJ
 ###############################################################################
 
 .. toctree::
-   :maxdepth: 3
+   :maxdepth: 4
    :hidden:
 
    about