docs: 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 )