Fix missing format when writing Algorithm-based objects

Added a writeFormat() method to Algorithm which must be called by the
write() method of derived classes.

modules/core/include/opencv2/core.hpp

@@ -3059,6 +3059,9 @@ public:
     /** Returns the algorithm string identifier.
      This string is used as top level xml/yml node tag when the object is saved to a file or string. */
     CV_WRAP virtual String getDefaultName() const;
+
+protected:
+    void writeFormat(FileStorage& fs) const;
 };
 
 struct Param {