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

add getDescription() to AirStart extension

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

@@ -29,6 +29,11 @@ public class AirStart extends AbstractSimulationExtension {
 		return name;
 	}
 
+	@Override
+	public String getDescription() {
+		return "Start simulation with a configurable altitude and velocity";
+	}
+	
 	public double getLaunchAltitude() {
 		return config.getDouble("launchAltitude", 0.0);
 	}