move AirStart extension files from .../impl/ to .../example

add getDescription() to AirStart extension

core/src/net/sf/openrocket/simulation/extension/example/AirStart.java

@@ -28,6 +28,11 @@ public class AirStart extends AbstractSimulationExtension {
 		name = L10N.replace(name, "{vel}", UnitGroup.UNITS_VELOCITY.toStringUnit(getLaunchVelocity()));
 		return name;
 	}
+
+	@Override
+	public String getDescription() {
+		return "Start simulation with a configurable altitude and velocity";
+	}
 	
 	public double getLaunchAltitude() {
 		return config.getDouble("launchAltitude", 0.0);