Add reference to event to tracks

[lobis]

Related to rest-for-physics/framework#260.

Adds reference to the event on each track on event initialization. This way we can access event information (and thus metadata) from the tracks, allowing to get information about volume / process names for print. This would also allow to do things such as access parent track from inside the child track itself, which can simplify some analysis logic.

The way this is setup is that these references are initialized on event load, requiring a loop over all tracks. In principle the added overhead should be small but if we notice it's too big the logic could be improved. The reason this reference update logic is not inside the track getter for the event, its because it would make this method no longer const, which I think it should be.