LArSoft

sim::OnePhoton is part of one of the two supported representation of scintillation light in LArSoft.
It is in fact part of the most expensive one, where each scintillation photon is saved individually as an element of a sim::SimPhotons collection.

Given that this is so expensive, it may be good to refresh and compact it.
The actions I propose are:

1. replacement of TVector3 with geo::Point_t in the two vector members
2. move of the bool data member at the end of the object

A survey in the code shows that the fixes after the first change are trivial.

Other actions that should take place at the same time:

1. ordering operator (ordering by increasing Time, then by MotherTrackID, InitialPosition (x, y, z) and FinalLocalPosition (x, y, z)
2. transformation into struct
3. removal of the empty constructor and explicit initialization of the data members