References may not be consistent

[arauschert]

Describe the bug

There are two concepts of references:

1. model_reference (reference to 3d model asset)
2. ExternalReference (which can reference also a 3d asset)

This may confuse the user.

For example in the RoadMarking element, there is no model_reference attribute, while for all other elements (traffic light, traffic sign, ground truth, moving object, stationary object) there are model_references.

A second example would be that the BoundingBox element also has a model_reference attribute, which duplicates the model_reference attribute of the parent elements (traffic light/sign, moving object...).

Furthermore, the road marking element of type stop line, where there is no direct link to the corresponding traffic sign / traffic light.

Describe the expected behavior

The question would be in references can be defined in a consistent way.

Show some screenshots