code: def faces(self, sites, permutation=None):
          
          if permutation is None:
              coords = [site.coords for site in sites]
          else:
              coords = [sites[ii].coords for ii in permutation]
          return [[coords[ii] for ii in f] for f in self._faces]
docs: Returns the list of faces of this coordination geometry. Each face is given as a
          list of its vertices coordinates.