Get rid of GDAL

[juliohm]

GDAL is a temporary dependency that we rely on to load specific GIS formats. It is very problematic, particularly on Windows, with multiple reports of unsuccessful builds.

Our ultimate goal for the near future is to get rid of this external library, and rely on native Julia backends only.

[juliohm]

GDAL is currently a direct dependency in order to

• Generate CRS strings (e.g. proj, wkt) to load/save files on disk
• Handle GeoPackage files (GeoPackage.jl is not there yet)
• Handle other less-popular formats (e.g. KML)

The plan is to fix (1) and (2), drop GDAL, and work on (3) with more time later.