Extract Geographic Raster metadata from recognized file types

[sblack-usu]

Reproduce HydroShare's metadata extraction in hsmodels. https://github.com/hydroshare/hydroshare/blob/develop/hs_file_types/models/geofeature.py#L338

Acceptance Criteria:
A geographic raster model can recognize special file types and extract metadata from these files into the a resulting instance of the model.

TODO:
Once a pattern is established here, implement metadata extraction for each aggregation type.