Guess cf coords before subsetting

[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.
• CHANGES.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?

• If one of "longitude" or "latitude" can't be found in the CF-compliant coordinates of the input dataset, spatial.subset will try to guess them ( cf-xarray does it really).

Does this PR introduce a breaking change?

No. Nothing is touched if coordinates are present. If they can't be found, it will fail silently and we can expect the subset to fail further down the line with some (hopefully) explicit message.

Other information

This allows the seamless usage of extract_dataset with datasets which lack proper attributes on their coordinates (our copy of ERA5, for example).