In CvBoost, delegated update_weights's implementation to a helper method.

This allows subclasses of CvBoost to override initial weights that
update_weights uses without duplicating its entire implementation.

modules/ml/include/opencv2/ml/ml.hpp

@@ -1253,6 +1253,8 @@ public:
 
 protected:
 
+    void update_weights_impl( CvBoostTree* tree, double initial_weights[2] );
+
     virtual bool set_params( const CvBoostParams& params );
     virtual void update_weights( CvBoostTree* tree );
     virtual void trim_weights();