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/tree.cpp

@@ -1681,6 +1681,7 @@ void DTreesImpl::writeTree( FileStorage& fs, int root ) const
 
 void DTreesImpl::write( FileStorage& fs ) const
 {
+    writeFormat(fs);
     writeParams(fs);
     writeTree(fs, roots[0]);
 }