Fixed bug due to longstanding inconsistency in FGAirport
getter functions return types.
Durk Talsma: Fixed traffic record initialization bug that occured
when taxiing traffic was waiting for traffic on runway
I refactored the XML loading code out of FGAirportDynamics and
FGRunwayPreference. I also added a new class XMLLoader, which serves as a
facade to the loader functions. Further I changed FGRunwayPreference to just
keep a FGAirport ref, which is more concise and closer to the right(tm)
solution than storing the airport data a second time ;-)