Raise ValueError if get_gis_type results in UNKOWN_TYPE

[dcdenu4]

get_gis_type returns gdal.UNKOWN_TYPE (0) if the file cannot be opened as a raster or vector. This is inconsistent with the rest of the API including get_raster_info and get_vector_info which raise a ValueError in that circumstance. It's also not intuitive that a Microsoft Word file would return a GIS type of UNKOWN_TYPE. PGP is a GIS processing library that only works with valid GIS types.

Fixes #244

[dcdenu4]

@phargogh, I'd be curious your thoughts on keeping UNKNOWN_TYPE around as a constant in __init__.py. In PGP, get_gis_type is the only place it's used, and InVEST only uses it in HRA, directly related to a get_gis_type call.

[phargogh]

@dcdenu4 much to my surprise, GDAL does actually support drivers that are neither rasters nor vectors! So yes, let's keep the UNKNOWN_TYPE flag.

Here's my rationale:

from osgeo import gdal

for i in range(gdal.GetDriverCount()):
    try:
        metadata = gdal.GetDriver(i).GetMetadata()
    except Exception as error:
        print(error, 'ERROR')
    else:
        raster_capable = 'DCAP_RASTER' in metadata
        vector_capable = 'DCAP_VECTOR' in metadata
        if not raster_capable and not vector_capable:
            print(metadata['DMD_LONGNAME'])

which outputs:

Geographic Network generic file based model
Geographic Network generic DB based model

[phargogh]

Source code looks good to me; I'm just waiting for the remaining tests to finish. There were a couple of conda 500 internal server errors, which are fine and flaky.