Update central heat pumps

[amos-schledorn]

Closes # (if applicable).

Changes proposed in this Pull Request

• Geothermal-sourced central heat pumps are split into heat pump and source: investment costs are allocated based on DEA data on this split. I am assuming that O&M costs can be kept unchanged, since they are expressed in relative terms (% per capacity/year or EUR/capacity)
• Excess-heat sourced heat pumps are added.
• Water-sourced heat pumps are added. Seawater heat pump data from DEA are used as a proxy.
• I am using sea-water heat pumps as a basis for water-sourced heat pumps in general. The default DEA data refers to the heat-pump-part only, assuming seawater intake etc. to already exist. They recommend to use the upper data in the upper uncertainty range if this additional equipment is necessary. We follow this recommendation here, which requires some custom indexing and inter-/extrapolation.

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.

[fneum]

Add a release note with a warning about the changed coverage of cost of central geothermal-sourced heat pump entry, then it's good to go!