Initial interface for ReservationSystem

Added preliminary functions for ReservationSystem class
Revise project structure

Added new class PassengerDatabase
Add FlightManager

Forgot FlightManager header. Whoops.
Merge remote-tracking branch 'origin/master' into reservationsystem
New interface

Revised the interface according to discussion in 12/5 meeting
Merge remote-tracking branch 'origin/reservationsystem' into UI
Merge branch 'UI' of https://github.com/collinday/CSM20-Final-Project into UI