allow keeping existing capacities in sec from power (#4)

[martacki]

Contents:

Enable retaining existing conventional capacities added in the power only model (elec.nc) for sector coupeled applications (add them again in prepare_sector_network), if specified in the config file.

Other comments: Keep experimenting with git cherry-picking... this originates here: https://github.com/open-energy-transition/pypsa-eur/pull/4 Here's a log file of initial commits:

• allow keeping existing capacities in sec from power

• remove print statement

• Fixes to keep caps

• allow keeping existing capacities in sec from power

• remove print statement

• Fixes to prepare sector network to reflect existing conventional generations

• Modifications to add renewable capacities from custom powerplants file

• Update scripts/prepare_sector_network.py

• Update scripts/prepare_sector_network.py

• Fixing existing_efficiencies

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Closes # (if applicable).

Changes proposed in this Pull Request

Checklist

• [x] I tested my contribution locally and it seems to work fine.
• [x] Code and workflow changes are sufficiently documented.
• [x] Changed dependencies are added to envs/environment.yaml.
• [x] Changes in configuration options are added in all of config.default.yaml.
• [x] Changes in configuration options are also documented in doc/configtables/*.csv.
• [x] A release note doc/release_notes.rst is added.