fix bug reported in https://github.com/lammps/lammps/issues/1294

      elif line.startswith("Dihedrals"):

        parts = self._split_values(line)

        system['ndihedrals'] = int(self._get_pair(parts[0])[1])

        system['nangletypes'] = int(self._get_pair(parts[1])[1])

        system['ndihedraltypes'] = int(self._get_pair(parts[1])[1])

        system['dihedral_style'] = self._get_pair(parts[2])[1]

      elif line.startswith("Impropers"):

        parts = self._split_values(line)