autotest: show groundspeed in flightgear display

useful with wind

Tools/autotest/aircraft/Rascal/Rascal110-JSBSim-set.xml

@@ -134,6 +134,7 @@ dynamics model, and external 3D model.
              right.add("/apm/altitude");
              right.add("/apm/heading");
              right.add("/apm/airspeed");
+             right.add("/apm/groundspeed");
          });
         </script>
   </rascal>

Tools/autotest/aircraft/Rascal/Systems/airdata.nas

@@ -64,6 +64,9 @@ var update_vars = func( dt ) {
     setprop("/apm/elevator", round100(getprop("/surface-positions/elevator-pos-norm"))); 
     setprop("/apm/rudder",   round100(getprop("/surface-positions/rudder-pos-norm"))); 
     setprop("/apm/throttle", round10(getprop("/engines/engine/rpm")));
+
+    setprop("/apm/groundspeed", round10(0.514444444*getprop("/instrumentation/gps/indicated-ground-speed-kt")));
+
     # airspeed-kt is actually in feet per second (FDM NET bug)
     setprop("/apm/airspeed", round10(0.3048*getprop("/velocities/airspeed-kt")));
 }