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/features2d/src/akaze.cpp

@@ -217,6 +217,7 @@ namespace cv
 
         void write(FileStorage& fs) const
         {
+            writeFormat(fs);
             fs << "descriptor" << descriptor;
             fs << "descriptor_channels" << descriptor_channels;
             fs << "descriptor_size" << descriptor_size;