code: def _get_supercell_size(self, s1, s2):
          
          if self._supercell_size == 'num_sites':
              fu = s2.num_sites / s1.num_sites
          elif self._supercell_size == 'num_atoms':
              fu = s2.composition.num_atoms / s1.composition.num_atoms
          elif self._supercell_size == 'volume':
              fu = s2.volume / s1.volume
          else:
              try:
                  el = get_el_sp(self._supercell_size)
                  fu = s2.composition[el] / s1.composition[el]
              except:
                  raise ValueError('Invalid argument for supercell_size.')
  
          if fu < 2/3:
              return int(round(1/fu)), False
          else:
              return int(round(fu)), True
docs: Returns the supercell size, and whether the supercell should
          be applied to s1. If fu == 1, s1_supercell is returned as
          true, to avoid ambiguity.