AC_WPNav: add clear_pilot_desired_acceleration

This allows quickly clearing out the pilot desired acceleration for
loiter contoller.

libraries/AC_WPNav/AC_WPNav.h

@@ -69,6 +69,9 @@ public:
     /// set_pilot_desired_acceleration - sets pilot desired acceleration from roll and pitch stick input
     void set_pilot_desired_acceleration(float control_roll, float control_pitch);
 
+    /// clear_pilot_desired_acceleration - clear pilot desired acceleration
+    void clear_pilot_desired_acceleration() { _pilot_accel_fwd_cms = 0.0f; _pilot_accel_rgt_cms = 0.0f; }
+
     /// get_stopping_point - returns vector to stopping point based on a horizontal position and velocity
     void get_loiter_stopping_point_xy(Vector3f& stopping_point) const;