AP_Navigation: add method update_path

Signed-off-by: Rhys Mainwaring <[email protected]>

libraries/AP_Navigation/AP_Navigation.h

@@ -95,6 +95,16 @@ class AP_Navigation {
     // attitude/steering.
     virtual void update_level_flight(void) = 0;
 
+    // update the internal state of the navigation controller when
+    // the vehicle has been commanded with a path following setpoint, which
+    // includes the closest point on the path, the unit tangent to the path,
+    // and the curvature. This is the step function for navigation control when
+    // path following. This function is called at regular intervals
+    // (typically 10Hz). The main flight code will call an output function
+    // (such as nav_roll_cd()) after this function to ask for the new required
+    // navigation attitude/steering.
+    virtual void update_path(const class Location &position_on_path, Vector2f unit_path_tangent, float path_curvature, int8_t direction) {}
+
     // return true if we have reached the target loiter location. This
     // may be a fuzzy decision, depending on internal navigation
     // parameters. For example the controller may return true only