Deprecate GeometryCollection in favor of FeatureCollection (e.g. DelayedVector)

[soxofaan]

see Open-EO/openeo-processes#253:

In VITO backend code we use GeometryCollection as a collection of separate geometries, for which separate aggregations should be calculated for example. In Open-EO/openeo-processes#253 there is discussion to deprecate GeometryCollection usage as much as possible and promote FeatureCollections instead.

In DelayedVector (and other places) we do things the other way around at the moment: when we get a FeatureCollection, we convert it to a GeometryCollection.

Changing this will probably cause some nuisance, e.g. in the interaction between geopyspark driver and in UDFs that depend on this behavior.

[soxofaan]

FYI: Open-EO/openeo-processes#253 has been merged, so now it's "official" that GeometryCollections have to be considered to be a single geometry, and that FeatureCollections have to be used to aggregate over separate geometries.

[soxofaan]

(internal ticket/ref: EP-3893)

[soxofaan]

Also see: Open-EO/openeo-processes#270

[soxofaan]

related to EP-3981 (add vector cube support) too

[soxofaan]

related: MultiPolygon should also not be (ab)used as a collection of geometries: #288