fix: multiindex future warning

[coroa]

Changes proposed in this Pull Request

Accounts for the API changes introduced by xarray v2023.08.0 (released August 20, 2023), via PR #7368 — "Expose Coordinates as part of Xarray's public API".

The index argument to convert_and_aggregate can now accept an xr.Coordinates object with a single dimension, and converts the previously used pd.Index or pd.MultiIndex objects correctly.

This contains changes from #500 , because i wanted to test it with PyPSA Eur and was unable to unless those were included. If we merge after it, that should be fine.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[coroa]

Rebased ontop of the master branch after #500 was merged.

[lkstrp]

Assert and import sorting will be changed again by #492

But let's get this in as well and do a release