FlightTasks: replace setpoint setters with members

I realized that instead of using the setpoint setters inline
in real world tasks everyone started to have its own setpoint
member variable and only call the setter in the end for all the
privatly generate setpoints. This makes the setter useless and
therefore I switched to member setpoints in the architecture.
They bring more felxibility which is obviously needed but also
less structure which is the price to pay.

src/lib/FlightTasks/FlightTasks.cpp

@@ -15,7 +15,7 @@ bool FlightTasks::update()
 	return false;
 }
 
-const vehicle_local_position_setpoint_s &FlightTasks::getPositionSetpoint()
+const vehicle_local_position_setpoint_s FlightTasks::getPositionSetpoint()
 {
 	if (isAnyTaskActive()) {
 		return _current_task->getPositionSetpoint();