- added UPstream::allGatherValues() with a direct call to MPI_Allgather.
  This enables possible benefit from a variety of internal algorithms
  and simplifies the caller

    Old:
        labelList nPerProc
        (
            UPstream::listGatherValues<label>(patch_.size(), myComm)
        );
        Pstream::broadcast(nPerProc, myComm);

    New:

        const labelList nPerProc
        (
            UPstream::allGatherValues<label>(patch_.size(), myComm)
        );

- Pstream::allGatherList uses MPI_Allgather for contiguous values
  instead of the hand-rolled tree walking involved with
  gatherList/scatterList.

-
- simplified the calling parameters for mpiGather/mpiScatter.

  Since send/recv data types are identical, the send/recv count
  is also always identical. Eliminates the possibility of any
  discrepancies.

  Since this is a low-level call, it does not affect much code.
  Currently just Foam::profilingPstream and a UPstream internal.

BUG: call to MPI_Allgather had hard-coded MPI_BYTE (not the data type)

- a latent bug since it is currently only passed char data anyhow