Mpi fixes

Fixes latest bug in #16.

Was generating tags for the sends/receives directly from the cell pointer addresses (they need to be per-cell unique identifiers). Turns out this was not a particularly robust solution as the maximum ID value is bounded to < 30 bits.

Switched to using a global counter, seems to work fine now.

See merge request !9


Former-commit-id: 44c49df999e95f23176a91abadf0e6abc466fabc