Fields local is_head not always set for ik=is=1 in box mode

[David Dickinson]

When nproc > naky * ntheta0 it appears that the is_head flag for the ik=is=1 supercell is not set on any processor in box mode. This is probably mostly fine as we do not expect to do any work for this supercell in box mode, but it would perhaps be better if we can assign ownership to a process even if there’s no work to do.