Update get_warming_level

[aulemahal]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• This PR does not seem to break the templates.
• HISTORY.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

Updates and modifications to get_warming_level:

• driving_institution is never needed. If "model" has not matched any column as-is, it tries again with requested_model.endswith(model).
• Change of the internal data to a list of dicts. Thus dicts are added to accepted inputs.
• List inputs can be of mixed types (not really useful, but a cool side-effect)
• Matches are done field by field, instead of the whole model name at once
• A few esthetical and nitpicky code changes
• If the output is a dict, the key is the name of the selected model, not the requested string.

Does this PR introduce a breaking change?

Maybe this last point is breaking... If two requested models match to the same column, then the output doesn't have the same length as the input.

Other information:

I think this means we can drop driving_institution from the official columns. (We could keep it in our CORDEX catalogs though).