Add geothermal central heating

[amos-schledorn]

Closes # (if applicable).

Changes proposed in this Pull Request

• This PR adds geothermal district heating (central geothermal-sourced heat pump).
• Data is based on DEA technology catalogue for geothermal district heating at a depth of 1200m, with no direct supply from the well to the district heating system. Other alternatives are available but this seems to be the most widely applicable, since for non-Danish systems, temperatures might very often be too high for direct usage of geothermal heat without boosting.
• Note that 1200m is rather shallow. For PyPSA-Eur, we might want to increase CAPEX slightly to achieve wider applicability. Happy to already do that here, but I'm afraid this could be misleading to those only checking the output tables.

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.