use `0` as tag in MPI to avoid too large value

Some versions of OpenMPI have a lower limit than 12345678; the MPI standard only guarantees values up to at least 32768 are valid.

Closes: #19 (closed)

dune/grid-glue/common/ringcomm.hh

@@ -221,7 +221,7 @@ namespace Parallel {
           std::array<MPI_Request,N> requests_send;
           std::array<MPI_Request,N> requests_recv;
 
-          int tag = 12345678;
+          int tag = 0;
           Dune::Hybrid::forEach(indices,
             // [&](auto i){
             //   MPI_SendVectorInRing(