Added GDAL raster driver support

Based on Daniel Wiesmann's work.
One last little change: Could we remove the errorchecking on the get_ds_geotransform. I realized that not all raster formats support this and the GDALGetGeoTransform  function returns a default value if the gt has not been previously set or is not supported by the datatype. I think errcheck is not necessary here. The default value is [0.0, 1.0, 0.0, 0.0, 0.0, -1.0]

So I would suggest:

`get_ds_geotransform = void_output(lgdal.GDALGetGeoTransform, [c_void_p, POINTER(c_double * 6)], errcheck=False)`