add direct (test?) dependency on GDAL

[visr]

See JuliaGeo/GDAL.jl#161 (comment)

[rafaqz]

Turns out this isnt so easy. GDAL is not available because we load ArchGDAL in an extension.

I guess we need to add a separare GDAL extension for the methods that use it, which will just load automatically when people load ArchGDAL?

It could be better to move those few methods to ArchGDAL instead?

[visr]

Hmm yeah I don't know about those methods, but moving them to ArchGDAL sounds like a better option.

[rafaqz]

Its things like GDAL.gdalgetrasternodatavalue which could totally be in ArchGDAL.

[rafaqz]

I removed the Aqua test here so the GDAL tests will at least work.