- mp_world and world_comm should be used only in MPI initialization,
  not directly in codes - replaced by mp_images and intra_image_comm
  (except in a few cases whose usage is unclear to me)
- mp_global should be used only to provide initialization routines
  mp_startup, mp_global_end
- specific mp_* modules should be used for parallelization level on *
  (* = images, pools, bands, orhtoh, ...)
Nothing actually changes (I hope) but things will look cleaner if the module
for the required parallelization level is USEd instead of a generic one that
does too meny things at the same time. The final goal of this operation is to
better disentangle the various parallelization levels and to have a better
organized, more granular MPI initialization that does not introduces artificial
and meaningless dependencies. Not done for: PHonon, CPV, TDDFPT, GWW, EPW