MPI TAG overflow restricts number of submeshes.

[bremerm31]

This is an issue with the mpi tags. The MPI spec only requires that up to 15-bits of tags be supported. This has led to each MPI implementation coming up with their own supported number of tags, e.g. Intel supports 16 bits, Mpich supports 21 bits, etc.

@jgbarbosa was kind enough to supply us with a temporary fix #148 This fix I believe limits us to 256 submeshes per locality.

One possibility is to create groups and use the tags as ids for the sub-meshes.