[pylink][bugfix] Fix internal output data storage

The data was dynamically attached to the class object itself. This data got lost when serializing the object state. Now we make an instance of the nested class and fill it with the data as it should be. Therefore the data can be serialized.

Also makes OutputData a dataclass (essentially some syntactic sugar which generates a matching constructor).

If model training and postprocessing are separated this bug should always come up.