Determine `activity` from `exp`

[zklaus]

In CMIP6 the activity_id that is part of the path is determined by the experiment_id. Hence, I suggest we determine it from that information in a similar way as we already find institutes from source_ids, thus making it optional to specify them explicitly in the recipe.

[mattiarighi]

Can we add this as a dictionary in config_developer (like institutes)?

[zklaus]

Good idea! I don't know what else is necessary in terms of backend mechanics.

[mattiarighi]

It should only affect _data_finder.py.

[zklaus]

This is essentially addressed in ESMValGroup/ESMValTool#1121 so when that PR is migrated and merged, this can be closed.