code: def to_unit_cell(self, in_place=False):
          
          frac_coords = np.mod(self.frac_coords, 1)
          if in_place:
              self.frac_coords = frac_coords
          else:
              return PeriodicSite(self.species, frac_coords, self.lattice,
                                  properties=self.properties)
docs: Move frac coords to within the unit cell cell.