refine GeopysparkCubeMetadata

[bossie]

A couple of issues:

• it's unclear why GeopysparkCubeMetadata has spatial_extent and temporal_extent in addition to those in its dimensions;
• it defines filter_bbox and filter_temporal methods whereas a similar filter_bands is defined in the CubeMetadata parent class instead;
• timestamps are stringly typed instead of e.g. datetime.datetime.

[soxofaan]

Another thing that could be considered here: GeopysparkCubeMetadata currently subclasses from CollectionMetadata, but with Open-EO/openeo-python-client#464 that should be changed to CubeMetadata