Initial interface for ReservationSystem

Here are my initial ideas for the ReservationSystem functions. Everything is fairly straightforward, but I would like to discuss the use of Passenger/FlightSet. The reason I did it this way is because it allows the client to decide what to do with the returned data instead of just directly sending it to cout. This is the way I imagine this working:
```C++
// In the client
ReservationSystem rs;
PassengerSet *allPassengers = rs.getPassengers();
cout << allPassengers->at(0); // ex: display the first result
```
Also, I didn't pay much attention to the constructor. I'm thinking pass it two filenames (the passengers/flights).