[GRASS-dev] PROJ6+ / WKT2 support

[Markus Metz]

Dear all,

I have prepared a rather large PR [0] to include proper support for PROJ6+
and WKT2.

The reason is that PROJ6+ uses a new mechanism based on spatial reference
id (authority name and code) and WKT2 for coordinate transformations. The
good old GRASS projection definition, a derivate of PROJ4-style definition,
is not able to capture details of all currently available coordinate
reference systems. With the recent substantial enhancements in PROJ, it is
time for GRASS to use these new enhancements. Current support for PROJ6+ in
G78 is not complete and still contains some bugs.

This PR (together with previous commits) would fix bugs also present in G78:
CRS mismatch [1] between input and output when creating a location from
some GDAL/OGR recognized input, importing to this location and exporting
again. The CRS should be exactly preserved. This bug happens when GRASS is
compiled with PROJ6+ and GDAL3.

A bug in PROJ can cause PROJ to select a wrong coordinate operation with a
datum transformation grid that does not cover the whole area of interest.
In these cases, reprojection fails. This PR takes care of this bug.

Fixing these bugs in G78 requires that all changes related to the new SRID
and WKT2 handling including all affected import and export modules need to
be backported.
Therefore I want to backport all relevant changes from master to G78 in
order to fix these bugs. These changes do not modify existing function
definitions, but add new functions to the GRASS headers.

Please let me know if you have objections regarding backporting to G78!

Markus M

[0] https://github.com/OSGeo/grass/pull/1240
[1] https://github.com/qgis/QGIS/issues/18596#issuecomment-753481210
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