Added a preprocessor hook for the SDMX transform

When the core data model has changed, we need a way to transform the SDMX document from the previous model to the new model for the backward compatibility. This commit adds `to_sdmx` method to `DataModelSerializers` class so that a preprocessor for the SDMX transform can be added per model basis.
Fix for the scope issue regarding the lambda function in `DataModelSerializers` class

The variable `unbound_func` in the lambda function of `attach_to_data_model` method of `DataModelSerializers` class took the value from the surrounding scope. This made every overwritten serializer bound to the last value of `unbound_func`. This commit fixes this problem.