AC_Avoidance: constify get_position

libraries/AC_Avoidance/AC_Avoid.cpp

@@ -289,7 +289,7 @@ void AC_Avoid::limit_velocity(float kP, float accel_cmss, Vector2f &desired_vel,
 /*
  * Gets the current xy-position, relative to home (not relative to EKF origin)
  */
-Vector2f AC_Avoid::get_position()
+Vector2f AC_Avoid::get_position() const
 {
     const Vector3f position_xyz = _inav.get_position();
     const Vector2f position_xy(position_xyz.x,position_xyz.y);

libraries/AC_Avoidance/AC_Avoid.h

@@ -85,7 +85,7 @@ private:
     /*
      * Gets the current position, relative to home (not relative to EKF origin)
      */
-    Vector2f get_position();
+    Vector2f get_position() const;
 
     /*
      * Computes the speed such that the stopping distance