Return L-SKEW weighting for donor catchment.

        Methodology source: Science Report SC050050, eqn. 6.19 and 6.22b

def _l_skew_weight(self, donor_catchment):
        """
        Return L-SKEW weighting for donor catchment.

        Methodology source: Science Report SC050050, eqn. 6.19 and 6.22b
        """
        try:
            dist = donor_catchment.similarity_dist
        except AttributeError:
            dist = self._similarity_distance(self.catchment, donor_catchment)
        b = 0.0219 * (1 - exp(-dist / 0.2360))
        c = 0.2743 / (donor_catchment.record_length - 2)
        return 1 / (b + c)