code: def obfn_dfd(self):
          r
          XF = sl.rfftn(self.obfn_fvar(), mp_Nv, mp_axisN)
          DX = np.moveaxis(sl.irfftn(sl.inner(mp_Df, XF, mp_axisM),
                                     mp_Nv, mp_axisN), mp_axisM,
                           self.cri.axisM)
          return np.sum((self.W*(DX-self.S))**2)/2.0
docs: r"""Compute data fidelity term :math:`(1/2) \| W \left( \sum_m
          \mathbf{d}_m * \mathbf{x}_m - \mathbf{s} \right) \|_2^2`.