FGAIMgr now searches for proximity to airports with logical networks defined at start...

FGAIMgr now searches for proximity to airports with logical networks defined at startup and periodically thereafter, and only starts AI traffic when a suitable airport is found in range (currently only KEMT).  In theory it should now be possible for a user to drop in a *.taxi file in my undocumented-shortly-to-change format into fgfsbase/ATC and have an AI Cessna appear at that airport and fly a circuit if they start there or fly near there.  In practice its probably better until we start using the output from Bernie's taxiway editor, and there's bound to be glitches once other networks are tried