Extension of traffic lights and signs regarding model references

[ThomasNaderBMW]

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Beside the model_references in stationary and moving object, it can be helpful to have a reference packed within traffic lights and traffic signs (main sign + supplementary sign).
The model_reference can be used e.g. to tell which 3D-model should be visualized.

Some questions to ask:

• What is this change?
  Added three model_references

• What does it fix? Is this a bug fix or a feature? Does it break any existing functionality or force me to update to a new version?
  It's a new feature the helps to use osi for visualization.
  How has it been tested?
  It works, testing was company internally.

Mention a member

@pmai , @engelben
This would be a PR for the planned CCB.

Check the checklist

• My code and comments follow the style guidelines and contributors guidelines of this project.
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests / travis ci pass locally with my changes.

[tbleher]

Makes sense to me, since it makes traffic signs and lights consitent with other objects.

[pmai]

Looking good for me. See also #408 for related changes.

[kmeids]

Output from CCB meeting - 25.11.2020

Actions:
@paagkame, to check the documentation on the changes done under "Files changed".

[paagkame]

documentation review is done. Looks good. I have nothing to change.

[kmeids]

Output from CCB meeting - 09.12.2020
Actions:

1. Pull request is ready for merge, to be done either by @pmai or @ThomasNaderBMW.