include SGRefObject before SGObject for swig
drop SERIALIZABLE_DUMMY
add namespace to get Externalizable to work for real
make ref/unref known *before* including the class
Added StructuredModel::structured_labels_factory() to create the right SO labels class.
Prepared StructuredLabels' methods for overloading.
Overloading add_label, set_label, get_label, get_num_elements in MulticlassSOLabels.
Added own storage object for real labels.
Temporarily disabled ensure_valid calls.