Make agency the default and add it to the schedule if not already added

def SetDefaultAgency(self, agency, validate=True):
    """Make agency the default and add it to the schedule if not already added"""
    assert isinstance(agency, self._gtfs_factory.Agency)
    self._default_agency = agency
    if agency.agency_id not in self._agencies:
      self.AddAgencyObject(agency, validate=validate)