Create PR to include our version of get_clustering_from_busmap into pypsa

[LukasFrankenQ]

PyPSA-Eur does not have partial grouping of generation plants as a feature, meaning if CCGT should be grouped, then it will be grouped in all countries (buses). If we want to model generators individually, then generators are modelled individually in all countries (buses). Here, we only want to model individual generators in GB, and we would like to group the ones in Germany for instance.

Right now, we have a script _clustering_helpers.py that is a partial copy of a PyPSA Script pypsa/clustering/spatial.py with the additional feature of partial grouping of generators added. This is code duplication, and undesirable in the long term. To alleviate this, we should create a PR to PyPSA, and hence attempt to get the feature into the main branch of PyPSA.