[PROJ] RFC4: last chance to comment before motion

[Even Rouault]

Hi,

PROJ 6.3 being out, it is time to consider the next steps :-)

RFC4
https://github.com/rouault/PROJ/blob/rfc4_remote_and_geotiff_grid/docs/source/community/rfc/rfc-4.rst
is hopefully now in its final state. Number of changes have been added to it since
the initial version to synchronize it with the implementation, but nothing dramatically different.
You can the diff at https://github.com/rouault/PROJ/compare/5bbfc07..2b16de3

It is now backed by a working implementation available at
https://github.com/OSGeo/PROJ/pull/1817

The CDN, storing grids converted to GeoTIFF, is now available at https://cdn.proj.org/

As an example, the following will now work (using the OSTN15_NTv2_OSGBtoETRS.tif grid):

$ echo 60 2 | PROJ_NETWORK=ON cs2cs -d 8 OSGB36 ETRS89
59.99957003	1.99763761 0.00000000

On next invocations, the parts of the grid that are needed and have already been
queried will be retrieved from the cache (at ~/.local/share/proj/cache.db on Linux)

For now, the RFC4 implementation works with a grid_alternatives database table still
pointing at the NTv2 & GTX files of https://github.com/osgeo/proj-datumgrid and it will
automagically try to fetch the correspond .tif files from the CDN when they are not locally
available.

In a next step, we'll also need to decide how/when we switch to
https://github.com/osgeo/proj-datumgrid-geotiff (which is the master of cdn.proj.org) and
if we provide a single or multiple packages of it (current total size of files is 486 MB),
and/or a utility to retrieve as a post installation stage grids of interest from a given
bounding box / producer / country from the CDN.
I was initially thinking to a python utility, but there could have
issues with Python dependencies, so perhaps a binary using the new curl dependency would be
easier to deploy & use. This can be for follow-up discussions.

Even

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