some refactoring and code cleaning

[fneum]

Changes proposed in this Pull Request

Admittedly multiple changes in one, but hopefully uncontroversial for the most part.

Reviewer, tick off boxes you agree on.

• remove unused {network} wildcard
• [ ] add pre-commit hooks
• move conda environment files to envs folder
• remove cluster.yaml and groups in Snakefile. Clean up for now, can be redone if needed.
• remove sector-coupling components (CHP, heat pump etc) from config.yaml
• some refactoring (sort imports, trailing whitespaces, move functions above __main__ if possible
• remove broken code that has been commented out for ages

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• [ ] Newly introduced dependencies are added to environment.yaml and environment.docs.yaml.
• [ ] Changes in configuration options are added in all of config.default.yaml, config.tutorial.yaml, and test/config.test1.yaml.
• [ ] Changes in configuration options are also documented in doc/configtables/*.csv and line references are adjusted in doc/configuration.rst and doc/tutorial.rst.
• A note for the release notes doc/release_notes.rst is amended in the format of previous release notes.

[fneum]

Note: Merge #204 into this branch before merging all into master.

[FabianHofmann]

Another point could be the removal of the plot scripts, make_summary and build_country_flh