ENH: additional control and access methods for PstreamBuffers

- PstreamBuffers nProcs() and allProcs() methods to recover the rank
  information consistent with the communicator used for construction

- allowClearRecv() methods for more control over buffer reuse
  For example,

      pBufs.allowClearRecv(false);

      forAll(particles, particlei)
      {
          pBufs.clear();

          fill...

          read via IPstream(..., pBufs);
       }

  This preserves the receive buffers memory allocation between calls.

- finishedNeighbourSends() method as compact wrapper for
  finishedSends() when send/recv ranks are identically
  (eg, neighbours)

- hasSendData()/hasRecvData() methods for PstreamBuffers.

  Can be useful for some situations to skip reading entirely.
  For example,

      pBufs.finishedNeighbourSends(neighProcs);

      if (!returnReduce(pBufs.hasRecvData(), orOp<bool>()))
      {
          // Nothing to do
          continue;
      }
      ...

  On an individual basis:

      for (const int proci : pBufs.allProcs())
      {
          if (pBufs.hasRecvData(proci))
          {
             ...
          }
      }

  Also conceivable to do the following instead (nonBlocking only):

      if (!returnReduce(pBufs.hasSendData(), orOp<bool>()))
      {
          // Nothing to do
          pBufs.clear();
          continue;
      }

      pBufs.finishedNeighbourSends(neighProcs);
      ...

src/sampling/meshToMesh/meshToMeshParallelOps.C

@@ -304,8 +304,6 @@ void Foam::meshToMesh::distributeCells
     List<labelList>& procLocalFaceIDs
 ) const
 {
-    PstreamBuffers pBufs(Pstream::commsTypes::nonBlocking);
-
     points.setSize(Pstream::nProcs());
     nInternalFaces.setSize(Pstream::nProcs(), 0);
     faces.setSize(Pstream::nProcs());
@@ -317,6 +315,8 @@ void Foam::meshToMesh::distributeCells
     procLocalFaceIDs.setSize(Pstream::nProcs());;
 
 
+    PstreamBuffers pBufs(Pstream::commsTypes::nonBlocking);
+
     for (const int domain : Pstream::allProcs())
     {
         const labelList& sendElems = map.subMap()[domain];