ENH: improve separation of OpenFOAM and ThirdParty mpi-libraries

- drop FOAM_MPI_LIBBIN variable in favour of FOAM_MPI from which
  two different locations can be determined.

  Previously there was something like this:

      FOAM_MPI_LIBBIN = $FOAM_LIBBIN/openmpi-1.5.1

  We now instead determine the locations from the FOAM_MPI directly:

      FOAM_MPI=openmpi-1.5.1

          $FOAM_LIBBIN/$FOAM_MPI     -> OpenFOAM  mpi-related
          $FOAM_EXT_LIBBIN/$FOAM_MPI -> ThirdPary mpi-related

ENH: relocate ParaView libraries and plugins back under FOAM_LIBBIN

- as discussed with Mattijs

src/Pstream/gamma/Make/files

@@ -3,4 +3,4 @@ Pstream.C
 OPwrite.C
 IPread.C
 
-LIB = $(FOAM_MPI_LIBBIN)/libPstream
+LIB = $(FOAM_LIBBIN)/$(FOAM_MPI)/libPstream

src/Pstream/mpi/Make/files

@@ -3,4 +3,4 @@ UIPread.C
 UPstream.C
 PstreamGlobals.C
 
-LIB = $(FOAM_MPI_LIBBIN)/libPstream
+LIB = $(FOAM_LIBBIN)/$(FOAM_MPI)/libPstream