AC_PosControl: add set_alt_target_to_current_alt method

libraries/AC_AttitudeControl/AC_PosControl.h

@@ -108,6 +108,9 @@ public:
     ///     target will also be stopped if the motors hit their limits or leash length is exceeded
     void set_alt_target_from_climb_rate(float climb_rate_cms, float dt);
 
+    /// set_alt_target_to_current_alt - set altitude target to current altitude
+    void set_alt_target_to_current_alt() { _pos_target.z = _inav.get_altitude(); }
+
     /// get_alt_target, get_desired_alt - get desired altitude (in cm above home) from loiter or wp controller which should be fed into throttle controller
     /// To-Do: remove one of the two functions below
     float get_alt_target() const { return _pos_target.z; }