ENH: improved isolation of MPI access in Pstreams

- refactor as an MPI-independent base class.

  Add bufferIPC{send,recv} private methods for construct/destruct.
  Eliminates code duplication from two constructor forms and reduces
  additional constructor definitions in dummy library.

- add PstreamBuffers access methods, refactor common finish sends
  code, tweak member packing

ENH: resize_nocopy for processorLduInterface buffers

- content is immediately overwritten

STYLE: cull unneeded includes in processorFa*

- handled by processorLduInterface

src/Pstream/mpi/Make/files

@@ -1,6 +1,7 @@
-UOPwrite.C
-UIPread.C
-UPstream.C
 PstreamGlobals.C
+UPstream.C
+
+UIPstreamRead.C
+UOPstreamWrite.C
 
 LIB = $(FOAM_MPI_LIBBIN)/libPstream