return a sensitivity, as well as corresponding metadata, for a given
        measurement configuration. Indices start at zero.

def get_sensitivity(self, config_nr):
        """return a sensitivity, as well as corresponding metadata, for a given
        measurement configuration. Indices start at zero.
        """
        if self.assignments['sensitivities'] is None:
            self._check_state()
            if self.can_model:
                self.model(sensitivities=True)
        cids = self.assignments['sensitivities'][config_nr]
        sens_data = [self.parman.parsets[cid] for cid in cids]
        meta_data = [self.parman.metadata[cid] for cid in cids]

        return sens_data, meta_data