Simulation uses tracking action to control primaries. This is not really the right way and leads to incorrect data structure. Right now we have

"mc" -> [array] -> "tracks" -> {"track0", "track1", ...}
-> <primary data>
-> "hits"
where each item in the array corresponds to a primary and then each of "track0", "track1" etc corresponds to a track created by that primary

I think we should have

"mc" -> "tracks" -> {"track0", "track1", ...}
-> "primaries" -> []
-> "hits"
i.e. the MAUS Spill should really be the Geant4 Event and we should push multiple tracks onto G4Event, one for each primary. Changes the data structure (and hence interface with e.g. digitisation) so would appreciate comments.

Probably a few other things like the "track0", "track1", etc should become an array.