Simplify DCS integration

Integration is suppressed by setting the Propagate integrators all to eNone at init. We then inject the state from set_current_state_body_axis which will no longer be overwritten by Propagate.

This removes the need to disable modules (JSBSim-Team/jsbsim#1481). GroundReactions now runs but we keep it effectively disabled by never extending the gear and by having no contact points.

EarthPositionAngle is forced to zero every frame because the DCS World's world is static and does not rotate and not doing this would probably cause longitude to drift west.

In set_current_state_body_axis the attitude is now set before the body velocity. SetUVW converts body velocity to inertial using the transforms in force and then Propagate::Run's CalculateUVW inverts that using the transforms. Setting the attitude first makes both directions use the same frame.

Because this now looks like finished, validated code I have taken the leap to strip all the commented-out FlightGear interface code carried in JSBSim_interface since the port.

Author: Zaretto