Review _create_grid() method

I updated to base class' _create_grid() method to support padding and to return the resulting grid's dimensions. This will allow Beirle and Fioletov to use it instead of providing their own implementation.

I.e., from

gp_sources, min_long, max_long, min_lat, max_lat, nlon, nlat = self._get_source_grid_settings(region, add_region_offset)

you can now move to

gp_sources, (nlon, nlat) = self._create_grid(region, resolution_lon, resolution_lat, padding=(add_region_offset[0], add_region_offset[1])
min_long, min_lat, max_long, max_lat = gp_sources.total_bounds