AP_Navigation: Add a loiter radius interface

libraries/AP_Navigation/AP_Navigation.h

@@ -56,6 +56,10 @@ public:
     // mission when approaching a waypoint
     virtual float turn_distance(float wp_radius, float turn_angle) const = 0;
 
+    // return the target loiter radius for the current location that
+    // will not cause excessive airframe loading
+    virtual float loiter_radius(const float radius) const = 0;
+
     // update the internal state of the navigation controller, given
     // the previous and next waypoints. This is the step function for
     // navigation control for path following between two points.  This