Returns the index of the reference bus.

def _get_reference_index(self, case):
        """ Returns the index of the reference bus.
        """
        refs = [bus._i for bus in case.connected_buses if bus.type == REFERENCE]
        if len(refs) == 1:
            return refs [0]
        else:
            logger.error("Single swing bus required for DCPF.")
            return -1