ENH: improved UPstream gather/scatter functionality

- 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

applications/test/parallel-comm2/Test-parallel-comm2.C

@@ -266,9 +266,8 @@ int main(int argc, char *argv[])
             UPstream::mpiGather
             (
                 myDigest.cdata_bytes(),     // Send
-                SHA1Digest::max_size(),     // Num send per proc
                 digests.data_bytes(),       // Recv
-                SHA1Digest::max_size(),     // Num recv per proc
+                SHA1Digest::max_size(),     // Num send/recv per rank
                 UPstream::commGlobal()
             );
         }

applications/utilities/surface/surfaceRedistributePar/surfaceRedistributePar.C

@@ -62,10 +62,10 @@ void writeProcStats
 )
 {
     // Determine surface bounding boxes, faces, points
-    List<treeBoundBox> surfBb(Pstream::nProcs());
-    surfBb[Pstream::myProcNo()] = treeBoundBox(s.points());
-    Pstream::gatherList(surfBb);
-
+    List<treeBoundBox> surfBb
+    (
+        UPstream::listGatherValues<treeBoundBox>(treeBoundBox(s.points()))
+    );
     labelList nPoints(UPstream::listGatherValues<label>(s.points().size()));
     labelList nFaces(UPstream::listGatherValues<label>(s.size()));