AD-6 propeller update

New code is in JSBSim now (but not yet in FlightGear) that allows you switch a prop into and out of constant speed mode. I've also added two optional tables that can be used to define compressibility effects on a prop. The AD-6 Skyraider prop has been updated. The AeroMatic values have been changed to suit the actual gear ratio of 2.28 (engine/prop). This results in a low-rpm prop. Great for cruising.