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

10 files changed, 90 insertions, 0 deletions

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: