Addressing edge case of already existing chp capacities

[toniseibold]

Closes PyPSA/pypsa-ariadne#250

Changes proposed in this Pull Request

The function add_chp_plants now iterates over every chp plant build before the first planning horizon and checks whether the link is already existing from the rule prepare_sector_network. If that's the case, the minimum capacity of the extendable link is adjusted to the installed capacity.
Since for German CHPs a efficiency depending on size and build year is passed, the efficiency is also adjusted for the extendable links. This is not the case for other European CHP plants.
A side effect of the iteration over all buses is avoiding the warnings WARNING:pypsa.components:Single value sequence for ... is treated as a scalar and broadcasted to all components. It is recommended to explicitly pass a scalar instead. in that function.

After executing the rule add_existing_baseyear the following installed capacities of CHPs are added for the year 2015-2020:

Checklist

• I tested my contribution locally and it works as intended.
• Code and workflow changes are sufficiently documented.
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• Changed dependencies are added to envs/environment.yaml.
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• Changes in configuration options are added in config/config.default.yaml.
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• Changes in configuration options are documented in doc/configtables/*.csv.
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• Sources of newly added data are documented in doc/data_sources.rst.
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• A release note doc/release_notes.rst is added.
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