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

Ansgar Burchardtrequested to merge
ansgar/dune-grid-glue:use-lower-tag-for-mpi into master

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)

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