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