set takeoff altitude

Initialize takeoff altitude at 20 meters above ground.

mavsdk_wrapper.cpp

@@ -347,6 +347,7 @@ int arm(void) {
 }
 
 int takeOff(void) {
+    action->set_takeoff_altitude(20);
     if (doAction(&Action::takeoff, "Takeoff failed"))
         return -1;