code: def _check_R2_axes_asym(self):
          
          for v in self.principal_axes:
              op = SymmOp.from_axis_angle_and_translation(v, 180)
              if self.is_valid_op(op):
                  self.symmops.append(op)
                  self.rot_sym.append((v, 2))
docs: Test for 2-fold rotation along the principal axes. Used to handle
          asymetric top molecules.