support runoff normalization for partial years with annual normalization data

[fneum]

Changes proposed in this Pull Request

It makes cutout.runoff(normalize_using_yearly=...) more lenient by supporting proportional scaling of partially represented years in the cutout, assuming even distribution of runoff throughout the year. In this case, a warning is printed due to the strong assumption.

Relevant for https://github.com/PyPSA/pypsa-eur/pull/1613 (e.g. July-June meteorological years, in case the original cutout does not span Jan-Dec for each year included)

This also means we can include hydro in the CI test of PyPSA-Eur, which uses only a week of weather data.

Checklist

• [x] Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• ~[] Unit tests for new features were added (if applicable).~
• ~[] Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).~
• [x] A note for the release notes doc/release_notes.rst of the upcoming release is included.
• [x] I consent to the release of this PR's code under the MIT license.