- simplifies communication structuring with intra-host communication.
  Can be used for IO only, or for specialised communication.

  Demand-driven construction. Gathers the SHA1 of host names when
  determining the connectivity. Internally uses an MPI_Gather of the
  digests and a MPI_Bcast of the unique host indices.

  NOTE:
    does not use MPI_Comm_splt or MPI_Comm_splt_type since these
    return MPI_COMM_NULL on non-participating process which does not
    easily fit into the OpenFOAM framework.

    Additionally, if using the caching version of
    UPstream::commInterHost() and UPstream::commIntraHost()
    the topology is determined simultaneously
    (ie, equivalent or potentially lower communication).