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/ml/src/em.cpp

@@ -770,6 +770,7 @@ public:
 
     void write(FileStorage& fs) const
     {
+        writeFormat(fs);
         fs << "training_params" << "{";
         write_params(fs);
         fs << "}";