AP_MotorsMulticopter: promote output_min from Matrix class

libraries/AP_Motors/AP_MotorsMulticopter.cpp

@@ -149,6 +149,14 @@ void AP_MotorsMulticopter::output()
     output_to_motors();
 };
 
+// sends minimum values out to the motors
+void AP_MotorsMulticopter::output_min()
+{
+    set_desired_spool_state(DESIRED_SHUT_DOWN);
+    _multicopter_flags.spool_mode = SHUT_DOWN;
+    output();
+}
+
 // update the throttle input filter
 void AP_MotorsMulticopter::update_throttle_filter()
 {

libraries/AP_Motors/AP_MotorsMulticopter.h

@@ -40,6 +40,9 @@ public:
     // output - sends commands to the motors
     virtual void        output();
 
+    // output_min - sends minimum values out to the motors
+    void                output_min();
+
     // output_to_motors - sends commands to the motors
     virtual void        output_to_motors() = 0;