Type issues

We assume wavefunctions are always distributed, and bands are never distributed, but groups of bands, called `slices', are sometimes used as local store (e.g. slice, slice_direction etc. in electronic) and sometimes global store (e.g. S_wvfn_slice in electronic). Thus we add a new logical bands_distributed to the wavefunction_slice type, set to `.false.' by default.

Work completed, tested and working.