ENH: add wrapped accessor for MPI_Comm

- UPstream::Communicator is similar to UPstream::Request to
  wrap/unwrap MPI_Comm. Provides a 'lookup' method to transcribe
  the internal OpenFOAM communicator tracking to the opaque wrapped
  version.

- provide an 'openfoam_mpi.H' interfacing file, which includes
  the <mpi.h> as well as casting routines.

  Example (caution: ugly!)

     MPI_Comm myComm =
         PstreamUtils::Cast::to_mpi
         (
             UPstream::Communicator::lookup(UPstream::worldComm)
         );

applications/test/parallel-comm2/Test-parallel-comm2.C

@@ -41,14 +41,7 @@ Description
 #include "PstreamReduceOps.H"
 #include "SHA1.H"
 
-// Include MPI without any C++ bindings
-#ifndef MPICH_SKIP_MPICXX
-#define MPICH_SKIP_MPICXX
-#endif
-#ifndef OMPI_SKIP_MPICXX
-#define OMPI_SKIP_MPICXX
-#endif
-#include <mpi.h>
+#include "openfoam_mpi.H"
 
 using namespace Foam;