def define_lattice_from_file( self, filename, cell_lengths ):
        """
        Set up the simulation lattice from a file containing site data.
            Uses `init_lattice.lattice_from_sites_file`, which defines the site file spec.
  
        Args:
            filename (Str): sites file filename.
            cell_lengths (List(x,y,z)): cell lengths for the simulation cell.

        Returns:
            None
        """
        self.lattice = init_lattice.lattice_from_sites_file( filename, cell_lengths = cell_lengths )