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.
Sarfraz A Nadeem