Enable constructing graphs from multiple pulsemaps

Created by: asogaard

In the context of e.g. IceCube Upgrade, it will be necessary to combine distinct pulsemaps (e.g. I3RecoPulseSeriesMapRFCleaned_mDOM and SRTInIcePulsesTWSRT), that cover disjoint sets of DOMs, into a single graph representing each event. The most direct way to do this would be to allow for multiple pulsemaps to be given as argument to SQLiteDatasethere and then concatenate the resulting features from each pulsemap.