Enable RAVEN to use Test SNEWS events; fixes #399

This MR allows Test SNEWS events (received weekly) to pass through the entire RAVEN pipeline, being caught in the publication conditions as to not trigger a real alert. This will allow regular testing to ensure the neutrino search aspect of RAVEN is working, important due to the low rate of SNEWS events (~ 1 / 100 years)

These changes also start paving the way to process MDC events by basing the determination of whether an event is a superevent or an external event by the presence of S in the graceid.