Encapsulate multi-process communication (part 2)

Podhorszki, Norbertrequested to merge
github/fork/bradking/mpi-encapsulation into master

Created by: bradking

Continue work started in #1691 to encapsulate multi-process communication behind a helper::Comm API:

  • Encapsulate remaining MPI types and constants inside helper::Comm and offer equivalents publicly.
  • Convert more code from direct MPI use over to helper::Comm. Add APIs to the latter as needed. * Abstract the helper::Comm implementation internally to work with either MPI or a dummy backend.
  • Replace MPI with helper::Comm in core ADIOS, IO, and Stream interfaces.
  • Minimize direct inclusion of ADIOSMPI.h to only places that still use MPI directly.

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