Fixes and code clean-up:

Fixes and code clean-up:

- Airports Directory
Thomas Foerster: Pulls out the FGTaxiNode implementation into gnnode.cxx.
Melchior / Durk: Copy Constructor and assignment operator for FGTaxiRoute

- AIModels Directory
Durk / Melchior / Czaba Halasz: Ensure that all derived classes use AIBase
member 'callsign'. Adapted, moved and deleted getter/setter functions where
necessary
Czaba Halasz: Fix AIBase model path vs. submodel path consistency.

- Traffic Directory and AIModels CreateFlightPlanCruise
DT: Temporary revert parts of the position estimation code.