Add standing loss param for thermal storage

[TomKae00]

Changes proposed in this Pull Request

• Added standing losses as parameters for PTES, central TTES, decentral TTES

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.

[cpschau]

LGTM and can be merged!

[fneum]

Perhaps one detail you could add: What is the unit? Not quite sure what the "per unit" refers to here.