ENH: Pstream specialization for float/scalar, FixedList (#2351)

- native MPI min/max/sum reductions for float/double
  irrespective of WM_PRECISION_OPTION

- native MPI min/max/sum reductions for (u)int32_t/(u)int64_t types,
  irrespective of WM_LABEL_SIZE

- replace rarely used vector2D sum reduction with FixedList as a
  indicator of its intent and also generalizes to different lengths.

  OLD:
      vector2D values;  values.x() = ...;  values.y() = ...;
      reduce(values, sumOp<vector2D>());

  NEW:
      FixedList<scalar,2> values;  values[0] = ...;  values[1] = ...;
      reduce(values, sumOp<scalar>());

- allow returnReduce() to use native reductions. Previous code (with
  linear/tree selector) would have bypassed them inadvertently.

ENH: added support for MPI broadcast (for a memory span)

ENH: select communication schedule as a static method

- UPstream::whichCommunication(comm) to select linear/tree
  communication instead of ternary or
  if (Pstream::nProcs() < Pstream::nProcsSimpleSum) ...

STYLE: align nProcsSimpleSum static value with etc/controlDict override

applications/test/parallel-communicators/Test-parallel-communicators.C

@@ -163,16 +163,6 @@ int main(int argc, char *argv[])
 
     if (Pstream::myProcNo(comm) != -1)
     {
-        //scalar sum = sumReduce(comm, localValue);
-        //scalar sum = localValue;
-        //reduce
-        //(
-        //    UPstream::treeCommunication(comm),
-        //    sum,
-        //    sumOp<scalar>(),
-        //    Pstream::msgType(),
-        //    comm
-        //);
         scalar sum = returnReduce
         (
             localValue,