extend TrackWriter to include shower frame
Instead of writing the tracks in the global coordinate system, alternatively they can be written in the shower frame, i.e. the coordinate system where one axis (z) is defined by the primary particle. In a homogeneous environment without magnetic fields, all showers would then look like vertical showers in the global coordinate system. In fact if the random number sequence is fixed they should be exactly the same. This can give us a handle on the precision of the geometry package, i.e. roations and boosts..