[PROJ] Switch to proj-datumgrid-geotiff for PROJ 7 ?

[Even Rouault]

> > It would probably be cleaner to avoid this patching and having
> > grid_alternatives directly point to .tif files. This would also enable us
> > to put https://github.com/osgeo/proj-datumgrid in a pure maintainance
> > state and just make https://github.com/osgeo/proj-datumgrid-geotiff the
> > new home for grids (currently the later has to be resynchronized with the
> > former)
> Possibly QGIS is a bit of an outlier in terms of Proj clients, but
> this change **would** impact us. Not in a major way, but in the order
> of 2-3 hours work to rectify (and even MORE proj version #ifdefs in
> the code! Woohoo!). If the intention is to make this an api break
> (given that it's v7), then that's fine, but if you're intending for
> this change to be transparent to clients just be aware that it isn't!

Maybe I've missed something or wasn't clear, but I don't see this is as an API 
break (as in C API). I can imagine some effect on test suites that would test 
that the PROJ pipeline from A to B would return a foo.gsb filename, and now 
that would be foo.tif, but not on the production code itself.
The impact as I see it would be more on the packaging side (grab the proj-
datumgrid-geotiff.zip )

>> One potential issue with switching to GeoTIFF files is for people having
>> pipeline strings using the current grid names in them. But similarly to 
what
>> is currently done for remote access, for local access, if we fail to find a
>> .gtx/.gsb file, we could just retry with the .tif extension.

> This would be a necessity for QGIS, otherwise existing projects will
> break and reproject differently upon loading in a build based on the
> newer PROJ version. (I suspect R would also see this as a necessity to
> avoid changing end user's existing results)

Yes, that would be definitely something to do to make the transition smoother.

Even

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