Refactor: Migrate MPI source from basic/ to communication/

Migrate MPI-related modules and source from basic/ to communication/, as discussed in issue #1043 This is part of the ongoing effort to reduce the size of the basic/ folder, and group low-level modules more meaningfully.

In addition, I have also:

• Renamed comm_inc.F90 to comm_allreduce_inc.F90, as nothing else is implemented in this file. Any additions can be placed in their own respective inc files, and still be made accessible via the comm.F90 module.
• Added a helper routine to compute the displacements array required for (all)gatherv and (all)scatterv. This is currently untested, but should be correct by inspection.
  • Unused routines trigger a warning that fails the foss warning builder. As such, I've removed this routine and will reintroduce in an MR where I use it