code: def _vec_alpha(self, donor_catchments):
          
          return np.dot(linalg.inv(self._matrix_omega(donor_catchments)), self._vec_b(donor_catchments))
docs: Return vector alpha which is the weights for donor model errors
  
          Methodology source: Kjeldsen, Jones & Morris 2014, eq 10
  
          :param donor_catchments: Catchments to use as donors
          :type donor_catchments: list of :class:`Catchment`
          :return: Vector of donor weights
          :rtype: :class:`numpy.ndarray`