BUG: Fix transform hierarchy errors after scene load
The problem was that qMRMLSceneTransformModel tries to determine the parent transform node as soon as the node is added to the scene, but at the time FirstTransformNode was added to the scene, the parent SecondTransformNode had not yet added to the scene, so it got NULL as parent transform.
In general, node IDs and references are not valid until the import is completed, therefore they cannot be added one by one.
Solution: do scene import always in batch mode.