Add StateAt functor and fix CLOSTESTTOBEAM

This adds a functor STATE_AT returning the tracks state requested by name from the state location enum. The CLOSESTTOBEAM functor utilises this now and thus actually always returns the CTB state or throws if not present. The old behaviour of CLOSTESTTOBEAM is now covered by TRACKSTATE which makes it more clear that it is the track defining which state is returned here.

Related to #392