More cleanup of tracking core.

Summary:

• 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::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).
• Cleaned up and refactored ca::TrackFinderWindow.
• Removed global triplets array in ca::TrackFinderWindow.

Supercedes !1899 (closed). See !1899 (closed) for additional details.