Set up indexing for updating v.

def _index_buses(self, buses):
        """ Set up indexing for updating v.
        """
        refs = [bus._i for bus in buses if bus.type == REFERENCE]
#        if len(refs) != 1:
#            raise SlackBusError
        pv = [bus._i for bus in buses if bus.type == PV]
        pq = [bus._i for bus in buses if bus.type == PQ]
        pvpq = pv + pq

        return refs, pq, pv, pvpq