fixed a bug where a HitBox trajectory could be re-iterated for a zero-length period of time
fixed a small bug which could result in unexpected behavior if setEndTime is called before other modifications to HitBox
re-ordered boolean operation for potential minor performance gain
refactoring so that changeInteractivity is not set when a group is changed
relaxed constraint on setting HitBox endtime, now allows endtime to equal current time (although endtime will have to be adjusted again before Collider can progress)