fix error message for 0D geometry variable in GeoDataset

[DirkEilander]

Issue addressed

Fixes #830

Explanation

Singe geometry geodatasets with zero dimensions are not discovered. While we can fix this it provides many other issues downstream as we also need a index dimension name which does not exist in this case. Hence I went with improving the GeoDataset typing and error messages.

Checklist

• Updated tests or added new tests
• Branch is up to date with main
• Tests & pre-commit hooks pass
• Updated documentation if needed
• Updated changelog.rst if needed
• For predefined catalogs: update the catalog version in the file itself, the references in data/predefined_catalogs.yml, and the changelog in data/changelog.rst

Additional Notes (optional)

Add any additional notes or information that may be helpful.

[hboisgon]

LGTM! And as mentioned in the issue happy with sel using a list as a workaround and only improving the error message here.