code: def distance(self, a, b):
          r
          
          return np.apply_along_axis(
              lambda vec: np.linalg.norm(vec, 1),
              1,
              self.Q * (a - b)
          )
docs: r"""
          Gives the distance between two model parameter vectors :math:`\vec{a}` and
          :math:`\vec{b}`. By default, this is the vector 1-norm of the difference
          :math:`\mathbf{Q} (\vec{a} - \vec{b})` rescaled by
          :attr:`~Model.Q`.
          
          :param np.ndarray a: Array of model parameter vectors having shape
              ``(n_models, n_modelparams)``.
          :param np.ndarray b: Array of model parameters to compare to, having
              the same shape as ``a``.
          :return: An array ``d`` of distances ``d[i]`` between ``a[i, :]`` and
              ``b[i, :]``.