Implement possibility of defining a grouped catalog collection entry

[santilland]

Currently the collections defined in the catalog file are expected to be individual collections, allowing adding multiple collections could support a use case where an "indicator" in the client is composed of multiple collections (this could be for different locations). The main question would be, what information is shown when loading the parent collection that references to the integrated ones

[santilland]

Maybe a "parent" collection can be defined with some metadata information, e.g.

collections:
  - parent_collection
    children:
      - child_collection_1
      - child_collection_2
  - other_collection
  - ...

[lubojr]

Similar approach as the "locations" concept in https://github.com/eurodatacube/eodash-catalog/blob/328b76abb7ac8ec58f0e6d9d33f6c07afc5babb7/collections/E13c_cruises_impact.yaml#L19 that does "area" filtering on a collection

[santilland]

If we assume that an added collection should only represent one location, which we can then "force" by applying filtering to an area, i think the Locations approach works very well. I have done an initial implementation in the multicollection branch https://github.com/eurodatacube/eodash-catalog/tree/multicollection