Ensure assignment of all regions without substation

[daniel-rdt]

Checklist

• I am using the latest release
• I am running on an up-to-date pypsa-eur environment. Update via conda env update -f envs/environment.yaml.

Describe the Bug

While building bus regions in build_bus_regions some onshore bus regions are filtered out for not having a substation. Fore some clusterings this leads to missing regions later on. They are, however, referenced for bus mapping since they still appear in the network.
This might be the cause for some issues with focus_weights.

Detail:
Here some regions for clustered onshore regions disappear:

pypsa-eur/scripts/cluster_network.py

Lines 437 to 443 in 03e1ed6

	for which in ("regions_onshore", "regions_offshore"):
	regions = gpd.read_file(getattr(input, which))
	regions = regions.reindex(columns=["name", "geometry"]).set_index("name")
	regions_c = regions.dissolve(busmap)
	regions_c.index.name = "name"
	regions_c = regions_c.reset_index()
	regions_c.to_file(getattr(output, which))

this is due to some regions that don't have a substation being filtered out here:

pypsa-eur/scripts/build_bus_regions.py

Line 138 in 03e1ed6

onshore_locs = n.buses.loc[c_b & n.buses.substation_lv, ["x", "y"]]

[fneum]

@FabianHofmann could you have a look?