Information contained in generated ROOT files

Currently our output ROOT files are upwards of 160MB for 1M events.

We should take a closer look at what data they actually contain - at a brief glance, the ROOT files defines both the Dalitz coordinates x and y, and also the center-mass energies. This is essentially duplicate information.

Similarly, based on a very brief investigation, the fields pAlpha, pAlpha.fUniqueID, and pAlpha.fBits seems to be identical for all events, and could maybe be removed.