Reuse WORLD and SELF.
Reuse WORLD and SELF.

This allows the --pdb mode to work properly on --single.
Previously the communicator is always freed, hence causing the debugging
to be very awkward. (comm.size would fail, e.g.)

rainwoodman/mpi4py_test,rainwoodman/runtests,rainwoodman/runtests,nickhand/runtests,rainwoodman/mpi4py_test,nickhand/runtests