Support standard voronoi clustering & "nearly-arbitrary" naming conventions

[davide-f]

Describe the feature you'd like to see

PyPSA-Earth-Sec has been built to work with alternative clustering that relies partially with 3-letter code standards.
It would be advisable to normalize the naming convention and generalize it to make it consistent with the power model.
Ideally, the code could be revised to support arbitrary naming convention in the form "{something}.{....}" or equivalent.

@doneachh I believe this may be related to #327 and #325.
Those may be 2 of the issues linked to here, not sure if they are the only ones.