Last vertex interaction splitting

[majacquet]

DO not merge, just for wheels.

Split at the last vertex of interaction if an interacting particles exits the patients.

To not have any bias created by the conditionnal probability to generate a track , the splitting method is the following :
if a primary particle exits the patient : kill and re create a track with a weight equal to the primary weight * 1/N weight, N being the multiplicity

if the track subsequently generated exits the patient, the total weight is incremented of the particle weight value, until it attains the weight of the primary particle. If the track does not exit the patient or undergoes an interaction, we kill it and relaunch a track without total weight incrementation.