AP_Motors: added disable_yaw_torque() method

this is used for tilt-vectored quadplanes to use only vectoring for
yaw control. This avoids some nasty transition issues

libraries/AP_Motors/AP_MotorsMatrix.cpp

@@ -969,3 +969,14 @@ void AP_MotorsMatrix::thrust_compensation(void)
         _thrust_compensation_callback(_thrust_rpyt_out, AP_MOTORS_MAX_NUM_MOTORS);
     }
 }
+
+/*
+  disable the use of motor torque to control yaw. Used when an
+  external mechanism such as vectoring is used for yaw control
+*/
+void AP_MotorsMatrix::disable_yaw_torque(void)
+{
+    for (uint8_t i = 0; i < AP_MOTORS_MAX_NUM_MOTORS; i++) {
+        _yaw_factor[i] = 0;
+    }
+}

libraries/AP_Motors/AP_MotorsMatrix.h

@@ -55,6 +55,10 @@ public:
     // using copter motors for forward flight
     float               get_roll_factor(uint8_t i) override { return _roll_factor[i]; }
 
+    // disable the use of motor torque to control yaw. Used when an external mechanism such
+    // as vectoring is used for yaw control
+    void                disable_yaw_torque(void) override;
+
 protected:
     // output - sends commands to the motors
     void                output_armed_stabilizing() override;

libraries/AP_Motors/AP_MotorsMulticopter.h

@@ -96,6 +96,10 @@ public:
         _thrust_compensation_callback = callback;
     }
     
+    // disable the use of motor torque to control yaw. Used when an external mechanism such
+    // as vectoring is used for yaw control
+    virtual void        disable_yaw_torque(void) {}
+
     // var_info for holding Parameter information
     static const struct AP_Param::GroupInfo        var_info[];