final mpiwrap / mpidummy bits

Podhorszki, Norbertrequested to merge
github/fork/germasch/pr/more_unified_mpi into release

Created by: germasch

This PR has three changes, which are actually independent of each other:

  • use MPI_COMM_NULL to keep track of having been invoked via serial API
  • remove unused MPI_File mpidummy implementation
  • minor cleanup in Transport.

Some more details are in the commit messages.

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