Reduce order of incoming external sky maps if too large; fixes #772

This MR reduces the size, whether multiordered or flattened, of external sky maps being ingested into gwcelery in order to prevent Kafka errors when sending a message with a resulting combined sky map. This comes after partners alerted that we may receive sky maps up to 10MB from different searches (IPN, NITRATES). This allows us to avoid demands on other experiments to make sky maps for our specific purposes and ingest whatever they send us.