code: def statexml2pdb(topology, state, output=None):
      
      state = Restart.from_xml(state)
      system = SystemHandler.load(topology, positions=state.positions)
      if output is None:
          output = topology + '.pdb'
      system.write_pdb(output)
docs: Given an OpenMM xml file containing the state of the simulation,
      generate a PDB snapshot for easy visualization.