Refactor: Allow for mutliple Tracker instances

[vsaw]

I have to confirm with a test suite, but I believe that the tracker does not differentiate merges trajectories without checking for object types.

So if e.g. a Person walks in front of a car, they will not be tracked as two separate objects but as one single object, even if the Modell is capable of detecting the objects separately.

[vsaw]

Looking at the code this requires to refactor the tracker to allow for different tracker instances. (Right now the tracker is a singleton instance).