AP_Math: added xy() method on Vector3

useful for getting xy length

libraries/AP_Math/vector3.cpp

@@ -617,6 +617,13 @@ 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

@@ -63,6 +63,9 @@
 template <typename T>
 class Matrix3;
 
+template <typename T>
+class Vector2;
+
 template <typename T>
 class Vector3
 {
@@ -189,6 +192,9 @@ 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();
+    
     // gets the length of this vector squared
     T  length_squared() const
     {