Add projsync utility

[rouault]

Fixes #1750

[hobu]

Why do we have projinfo and then proj_download? What discriminates the underscore separation? GDAL's behavior on this topic is so arbitrary that it requires lookup just about every time you use the tools. Let's not repeat that with PROJ.

[hobu]

I wonder if this should be called some variant of sync. It isn't a generic download tool. We could expand it to also include fetching latest DBs too, which would provide a lot of convenience.

[kbevers]

Looks good to me. This is a nice utility to add the package.

I wonder about the tests of the application, how resilient are they to changes in PROJ-data/the CDN? Do we need to update the tests everytime a change is made on the CDN or will this still work unchanged?

[rouault]

@hobu I was indeed midly satisfied with the name. projdownload seemed a bit stodgy to me, but projsync would be good. I'll go for it, if you and @kbevers is happy with that

Regarding synchronizing proj.db, this is a hard topic as I discussed a number of times. PROJ is highly sensitive to the content of proj.db. So if we were to do it, we would likely to have proj_7.0.0.db files compatible of PROJ 7.0.0 binary.

[rouault]

how resilient are they to changes in PROJ-data/the CDN?

They should be pretty resilient, unless we remove some files / source agencies.

[hobu]

So if we were to do it, we would likely to have proj_7.0.0.db files compatible of PROJ 7.0.0 binary.

My thought was to update if a new version of EPSG came out. Maybe it isn't worth the trouble.

[kbevers]

I'll go for it, if you and @kbevers is happy with that

Sounds good to me.

My thought was to update if a new version of EPSG came out. Maybe it isn't worth the trouble.

It seems that EPSG doesn't really follow semantic versioning, so an increase in the patch-number can add a new operation or some other thing that requires new code. #1869 is a good example of that. It would be great if we in the future could decouple the EPSG-registry from specific PROJ releases and update the database when new updates appear in the EPSG registry but we're not there yet.