- Cleaned up ca::TripletConstructor.

- Removed global ca::TrackFit instance from ca::TripletConstructor.
- Removed access to ca::InputData from ca::TripletConstructor (all needed data is already in ca::WindowData).
- Switched from global to window-based hit index in ca::Branch. Allows simplification of many interfaces
  (removal of ca::InputData).
- Cleaned up ca::TrackFinder.
- In ca::TrackFinder, replaced separate vector instances which store
  upper and lower bounds of windows by single vectors that store
  these bounds together as a pair.
- Cleaned up ca::TrackFinderWindow.