The attached patch adds a very thin layer of code to ExternalLibraries/MPI to provide access to two communicators
MPI_Comm_Universe which can be used for multi-model runs (or runs started via
The code does not itself split a communicator, it only provides routines to get and set the communicators.
It supports C and F90 calls (and C++ calls through external C and call from F77 files since Cactus compiles those with at a F90 compiler).
Documentation is provided in the tex file and in the grdoc comments.