ENH: allow disabling of initial MPI_Comm_dup(MPI_COMM_WORLD,...)

- can use -mpi-no-comm-dup to suppress the initial communicator
  duplication (to avoid potential deadlock with coupled processes).
  This is partly related to comments in merge-request !735

ENH: simplify parsing/removal of local -world option

- can extract the world name in a single pass and also makes the
  parsing robuster.

ENH: support regular MPI_Comm_split()

- the two-step method of Allgather + Comm_create_group may not be
  expected by other applications (issue #3127) and that can lead to
  deadlock, so also add in code for the regular MPI_Comm_split.
  Does not support re-sorting keys!

FIX: faulty logic for splitting communicators

- only affected more recent develop branch