remove gdal dependency and rely on rasterio+rasterio’s gdal binaries to simplify the code base and make installation easier

[rbavery]

It looks like rasterio binaries containing GDAL are available for MacOS, with other OSes stated to be a future work item: https://rasterio.readthedocs.io/en/latest/installation.html

this would simplify installation of solaris so that it doesn't depend on installing GDAL beforehand via conda or something else.

I'm not sure if this is currently possible or not for other OSes besides Mac, this issue looks a tad stale: rasterio/rasterio#942