add energy power ratio ltes

[TomKae00]

Closes # (if applicable).

Changes proposed in this Pull Request

• Added energy to power ratio calculations for water pit and tank storage
• Changes allow the integration of these storages into pypsa-eur as storage units.

Checklist

• [✓] Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• [✓] Data source for new technologies is clearly stated.
• [✓] Newly introduced dependencies are added to environment.yaml (if applicable).
• [✓] 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 GPLv3 license.

[amos-schledorn]

Looks great! Just a small suggestion for readability. Also, could you add an entry in the release notes?

[amos-schledorn]

Looks great, thanks!