New get time method at any point of the track

In this MR I have implemented a new .getTime(particle,u) method that gives back the time at any point in the track. This method needs a particle in order to be used because of the particle/track design.

One can get the time at the start of the track for u = 0 simply by using it like: track.getTime(particle,0)

And get the time at the end of the track for u = 1 by: track.getTime(particle,1)

I need to check a couple more things before this is ready for code review.