change from MPI_COMM_WORLD to custom mpiCommWorld. needs further testing...

First attempt to remove MPI_COMM_WORLD from Bonsai in favour of a custom mpiCommWorld communicator.

Todo:
 *  Split communicator n-ways when >1 ranks per nodes are found consistently for all jobs, so that we can n subcommunicators with 1 rank per node.
 * tests, tests, tests

Not yet for merging, but to track the changes..