don't set MPI communicator when compiling in serial with MPI stubs

  // whereas if partitions are not defined then world is equal to

  // MPI_COMM_WORLD.

#if !defined(MPI_STUBS)

  // plumed does not know about LAMMPS using the MPI STUBS library and will

  // fail if this is called under these circumstances

  p->cmd("setMPIComm",&world);

#endif

  // Set up units

  // LAMMPS units wrt kj/mol - nm - ps