AP_Math: make vector3 xy() method return a reference

libraries/AP_Math/vector3.cpp

@@ -617,13 +617,6 @@ bool Vector3<T>::segment_plane_intersect(const Vector3<T>& seg_start, const Vect
     return true;
 }
 
-// return xy components of a vector3
-template <typename T>
-Vector2<T> Vector3<T>::xy()
-{
-    return Vector2<T>{x,y};
-}
-
 // define for float and double
 template class Vector3<float>;
 template class Vector3<double>;

libraries/AP_Math/vector3.h

@@ -192,9 +192,15 @@ public:
     // rotate vector by angle in radians in xy plane leaving z untouched
     void rotate_xy(float rotation_rad);
 
-    // return xy components of a vector3
-    Vector2<T> xy();
-    
+    // return xy components of a vector3 as a vector2.
+    // this returns a reference to the original vector3 xy data
+    const Vector2<T> &xy() const {
+        return *(const Vector2<T> *)this;
+    }
+    Vector2<T> &xy() {
+        return *(Vector2<T> *)this;
+    }
+
     // gets the length of this vector squared
     T  length_squared() const
     {