Use GeoFeatureModelSerializer for ClimateDataCell serialization

[ddohler]

Currently, the ClimateDataCellBaseSerializer relies on a custom ClimateDataCellGeometrySerializer to serialize it as GeoJSON. All other endpoints that serialize points use GeoFeatureModelSerializer to serialize the geometry field directly. Switching the ClimateDataCellBaseSerializer to inherit from GeoFeatureModelSerializer would allow us to eliminate the custom ClimateDataCellGeometrySerializer.