Shmem comms [NO MPI] target added. The dwf test runs and passes.

Not really shaken out to my satisfaction though as I want more tests done, so don't declare as working.
But committing my current while I try a few experimentals.

lib/communicator/Communicator_mpi.cc

@@ -31,6 +31,15 @@ Author: Peter Boyle <paboyle@ph.ed.ac.uk>
 namespace Grid {
 
   // Should error check all MPI calls.
+void CartesianCommunicator::Init(int *argc, char ***argv) {
+  MPI_Init(argc,argv);
+}
+
+  int Rank(void) {
+    int pe;
+    MPI_Comm_rank(MPI_COMM_WORLD,&pe);
+    return pe;
+  }
 
 CartesianCommunicator::CartesianCommunicator(const std::vector<int> &processors)
 {