Remake combined skymap and rerun RAVEN pipeline when new sky maps are available; fixes #548

This MR adds the functionality that (after the initial re-running of RAVEN due to sky maps being initially available) any additional GW or external sky map will rerun the RAVEN pipeline and calculate a new combined sky map. If a combined sky map was already added to the superevent when sending a previous alert, this will also copy this new combined sky map to the superevent to replace it in the next alert.