[MPI] Check that MPI was initialized when constructing a CollectiveCommunication

This check was added to improve the error message returned when using
MPI without correctly initializing it (e.g. by creating a parallel
grid), see FS#1612 for further information.

dune/common/parallel/mpicollectivecommunication.hh

@@ -150,6 +150,10 @@ namespace Dune
       : communicator(c)
     {
       if(communicator!=MPI_COMM_NULL) {
+        int initialized = 0;
+        MPI_Initialized(&initialized);
+        if (!initialized)
+          DUNE_THROW(ParallelError,"You must call MPIHelper::instance(argc,argv) in your main() function before using the MPI CollectiveCommunication!");
         MPI_Comm_rank(communicator,&me);
         MPI_Comm_size(communicator,&procs);
       }else{