MPI: Improve reduction function
MPI: Improve reduction function

* dev/mpi/mpi_caf.c (get_MPI_datatype): Make static, handle
  complex 4+8.
  (co_reduce_1): Fix implementation; handle contiguous memory.

zbeekman/opencoarrays,sourceryinstitute/opencoarrays,sourceryinstitute/opencoarrays,sourceryinstitute/opencoarrays,jerryd/opencoarrays,zbeekman/opencoarrays,zbeekman/opencoarrays,jerryd/opencoarrays,jerryd/opencoarrays