code: def remove_non_ground_states(self):
          
          group_func = lambda e: e.composition.reduced_formula
          entries = sorted(self.entries, key=group_func)
          ground_states = set()
          for _, g in itertools.groupby(entries, key=group_func):
              ground_states.add(min(g, key=lambda e: e.energy_per_atom))
          self.entries = ground_states
docs: Removes all non-ground state entries, i.e., only keep the lowest energy
          per atom entry at each composition.