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/src/algorithm.cpp

@@ -57,7 +57,6 @@ void Algorithm::save(const String& filename) const
 {
     FileStorage fs(filename, FileStorage::WRITE);
     fs << getDefaultName() << "{";
-    fs << "format" << (int)3;
     write(fs);
     fs << "}";
 }
@@ -67,6 +66,11 @@ String Algorithm::getDefaultName() const
     return String("my_object");
 }
 
+void Algorithm::writeFormat(FileStorage& fs) const
+{
+    fs << "format" << (int)3;
+}
+
 }
 
 /* End of file. */