apply_polygon (previously known as chunk_polygon) abuses MultiPolygon as collection of polygons in its internal API

[soxofaan]

Stumbled on this while working on #287:

The apply_polygon process implementation (originally added as chunk_polygon in a411109) coerces the provided geometries into a MultiPolygon :

openeo-python-driver/openeo_driver/ProcessGraphDeserializer.py

Lines 859 to 874 in ea078ff

	# TODO: this logic (copied from original chunk_polygon implementation) coerces the input polygons
	# to a single MultiPolygon of pure (non-multi) polygons, which is conceptually wrong.
	# Instead it should normalize to a feature collection or vector cube.
	if isinstance(polygons, DelayedVector):
	polygons = list(polygons.geometries)
	for p in polygons:
	if not isinstance(p, shapely.geometry.Polygon):
	reason = "{m!s} is not a polygon.".format(m=p)
	raise ProcessParameterInvalidException(parameter="polygons", process="apply_polygon", reason=reason)
	polygon = MultiPolygon(polygons)
	elif isinstance(polygons, shapely.geometry.base.BaseGeometry):
	polygon = MultiPolygon(polygons)
	elif isinstance(polygons, dict):
	polygon = geojson_to_multipolygon(polygons)
	if isinstance(polygon, shapely.geometry.Polygon):
	polygon = MultiPolygon([polygon])

and pushes this API through into geopyspark driver, e.g.
https://github.com/Open-EO/openeo-geopyspark-driver/blob/1f22c0da34535e986908480673ad341c9f89324d/openeogeotrellis/geopysparkdatacube.py#L572-L575

MultiPolygon should not be abused as a collection of polygons. Instead the API should use something like a FeatureCollection or vector cube abstraction

[soxofaan]

related to (or causing) this confusion: the apply_polygon process should use argument geometries instead of polygons -> Open-EO/openeo-processes#511