FlightTaskManual: method to reset all members setpoints to NAN

src/lib/FlightTasks/tasks/FlightTaskManual.cpp

 /****************************************************************************
  *
- *   Copyright (c) 2017 PX4 Development Team. All rights reserved.
+ *   Copyright (c) 2018 PX4 Development Team. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -98,3 +98,12 @@ bool FlightTaskManual::_evaluateSticks()
 		return false;
 	}
 }
+
+void FlightTaskManual::_resetToNAN()
+{
+	_thr_sp *= NAN;
+	_vel_sp *= NAN;
+	_pos_sp *= NAN;
+	_yaw_sp = NAN;
+	_yaw_rate_sp = NAN;
+}