ENH: additional MPI gather/scatter routines, globalIndex gather methods

- UPstream::mpiGather (MPI_Gather)   - used by Pstream::listGatherValues
- UPstream::mpiScatter (MPI_Scatter) - used by Pstream::listScatterValues

  These are much simpler forms for gather/scatter of fixed-sized
  contiguous types data types (eg, primitives, simple pairs etc).

  In the gather form, creates a list of gathered values on the master
  process. The subranks have a list size of zero.

  Similarly, scatter will distribute a list of values to single values
  on each process.

  Instead of

      labelList sendSizes(Pstream::nProcs());
      sendSizes[Pstream::myProcNo()] = sendData.size();
      Pstream::gatherList(sendSizes);

  Can write

      const labelList sendSizes
      (
          UPstream::listGatherValues<label>(sendData.size())
      );

  // Less code, lower overhead and list can be const.

  For scattering an individual value only,
  instead of

      labelList someValues;
      if (Pstream::master()) someValues = ...;

      Pstream::gatherList(sendSizes);
      const label localValue
      (
          someValues[Pstream::myProcNo()]
      );

  Can write

      labelList someValues;
      if (Pstream::master()) someValues = ...;

      Pstream::gatherList(sendSizes);
      const label localValue
      (
          UPstream::listScatterValues<label>(someValues)
      );

   Can of course also mix listGatherValues to assemble a list on master
   and use Pstream::scatterList to distribute.

ENH: adjusted globalIndex gather methods

- added mpiGather() method [contiguous data only] using MPI_Gatherv

- respect localSize if gathering master data to ensure that a
  request for 0 master elements is properly handled.