Two options I've been thinking about:
* We could ask Heiko if he could provide us something to register unserializable variables (e.g. SGRefObject)
* Wrapping `m_outputs` into `StructuredLabels`, which leads to the next problem I did not realize in StructuredLabels::init():
  ```SG_ADD((CSGObject**) &m_labels, "m_labels", "The labels", MS_NOT_AVAILABLE);```
  Please note, that `m_labels` is no `CSGObject` any more.  So we lost serialization of `StructuredLabels` as well.

So we need to decide about both...