Gas, sector coupling and electrical clustering can not be run independently

[CarlosEpia]

When using spatial clustering, the argument args["network_clustering"]["active"] decides if the electrical AND gas clustering are activated. Therefore it is not possible to use one of them independently. In addition, the sector clustering has its own "active" parameter in the args, but it is only executed when the gas clustering is used.

There is a dirty provisional solution for using gas clustering alone. Set args["network_clustering"]["active"] = False initially and change it after the electrical clustering is skipped and before calling the gas clustering:

etrago.spatial_clustering()
etrago.buses_by_country()
etrago.geolocation_buses()
args["network_clustering"]["active"] = True
etrago.spatial_clustering_gas()

[KathiEsterl]

I started tackling this issue when starting to work on Issue #800.

You can see my changes here: b9d7eaa

Open ToDos:

• Check if only gas_clustering without AC_clustering runs correctly (it is running through, but I haven't checked if the results are plausible; there are quite many warnings in the gas_clustering)
• Separate the sector_coupled_clustering from the gas_clustering
  I would propose to add a new file cluster/other_sectors.py and shift the corresponding code. I would then also combine the electrical clustering, the gas clustering and the sector coupled clustering in one function etrago.spatial_clustering() (so remove etrago.gas_clustering()). Additionally, as soon as this is done, within the args the setting for the sector_coupled_clustering needs to be pulled out from ["network_clustering"["gas_grids"] to an own sub-category to make this independence clear.
• Separate the h2 from the ch4 clustering and pot. add new args like h2-busmap or h2-wight-csvs