Fix merge issue503

[JeromeCCP9]

Issue #503 contains two unrelated problems.

The parallelisation of the plot routine for generating xsf files was broken after a later merge that affected how the k-point distribution is handled (passed as an argument instead of being regenerated locally). This is corrected with a simple cycle condition and passing the k-distribution list.

A second, earlier commit that applied the ws_distance method to postw90 (mirroring its use in hamiltonian.F90) was apparently reverted in entirety (see discussion in issue #503 ); it is fixed by replaying those changes and merging.