Proj_jll grid files

[evetion]

Based on the discussion in #75 and my troubles to get the CI working for PROJ_NETWORK in evetion/GeoArrays.jl#126, we should try to document how downstream packages can make sure the PROJ grid files are available (remotely).

For example, GeoArrays does not depend on Proj or Proj_jll directly, only indirectly on the jll based on GDAL (via ArchGDAL). Currently I try to just PROJ_NETWORK=ON and hope for the best. Ideally, this works out of the box, like Proj_jll running projsync on init or automatically execute things like proj_context_set_search_paths into the package.

[evetion]

One of the reasons that this should work out of the box is that warping without Proj datagrids will just silently be wrong, as we have seen in other issues (#71).

[visr]

The only_best option from #87 should help with avoiding silently wrong issues. There was some talk about perhaps making that the default in a breaking PROJ release.