align DataCubeParameters with load_collection

[bossie]

https://github.com/Open-EO/openeo-geopyspark-driver/issues/812

[bossie]

load_stac now reuses dedicated method to create DataCubeParameters from LoadParameters. It's in a new module because otherwise there's a circular reference (layercatalog.py depends on load_stac.py).

This dedicated method makes use of some constants (in this case: EvalEnv keys) so to avoid another circular reference (layercatalog.py depends on datacube_parameters.py), these constants have been put into another module: constants.py.

[bossie]

Ok to put EvalEnv keys:

• in a class called EvalEnvKeys;
• in a module called constants.py?

or is there a more Pythonic way that you prefer @soxofaan ?

[bossie]

I aligned it with some other containers-of-constants in the code like YARN_STATE, JOB_STATUS etc.