load_stac: dimension names

[m-mohr]

Process ID: load_stac

Describe the issue:
The datacube that is generated from load_stac is not clearly defined, e.g. the dimension names are not set.

Proposed solution:

1. If cube:dimension is present, use it.
2. If loading from a datacube file format that provides unambiguous dimension names (e.g. netCDF), use it.
3. In all other cases, follow the guidelines in the API:
   

Additional context:
Related issues: #488, Open-EO/openeo-test-suite#9