bug #80, exception flying west from NZAA (Auckland, NZ). The underlying

bug #80, exception flying west from NZAA (Auckland, NZ). The underlying
issue is numerical problems in _wgs84_inverse, triggered by the traffic-manager
testing enormous distances geodetically.

Changes
 - refactor AISchedule::update, so the aircraft approximate position
   calculation is simpler.
 - refactor AIAircraft to no longer use exceptions as a return value mechanism
 - change AISchedule::update to use cartesian math for in-visible-range check