UEP supercell compatibility

[patrick-austin]

supercell, is silently unused by the UEP method (it would normally be used when generating the structures but no structure files are created and instead just the yaml needed for the UEP optimise step).

It is desriable to be able to save supercells of the structure when saving after clustering. The other methods can be made to (implicitly) cluster supercells due to the fact supercells were generated earlier on. To output supercells for UEP we would need to extend PyMuonSuite to use the supercell parameter at this stage of the process, iff the calculator is UEP (otherwise we would expand the cell twice). Alternatively an explicit parameter which only expands the cell during clustering could be introduced. This might however introduce confusion for the non-UEP methods.

We should also print/log a warning if supercell is provided when generating structures for the UEP method, since it is not actually used here unlike the other methods, which is inconsistent/confusing.