Some initial code for AI Aircraft making use of SIDs. Current code is a bit

Some initial code for AI Aircraft making use of SIDs. Current code is a bit
experimental: Only one SID per runway is supported, and the waypoints are
read from file at every request, which is not very efficient. The current
code is only executed when FlightGear is configured to use airport dynamics
data from the scenery repository, instead of the base package. Since the
latter is still the default, I believe that this will provide enough safe-
guarding to commit SID/STAR support in small, incremental steps.
Next step will be buffering and support for multiple departure routes per
runway.