Fixing multi_biorbd_model

[pariterre]

All Submissions:

• [x] Have you followed the guidelines in our Contributing document [docs/contribution.md]?
• [x] Have you checked to ensure there aren't other open [Pull Requests] for the same update/change?
• [x] Have you opened/linked the issue related to your pull request?
• [x] Have you used the tag [WIP] for on-going changes, and removed it when the pull request was ready?
• [x] When ready to merge, have you sent a comment pinging @pariterre in it?

New Feature Submissions:

1. [x] Does your submission pass the tests (if not please explain why this is intended)?
2. [x] Did you write a proper documentation (docstrings and ReadMe)
3. [x] Have you linted your code locally prior to submission (using the command: black . -l120 --exclude "external/*")?

Changes to Core Features:

• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] Have you written new examples for your core changes, as applicable?
• [x] Have you written new tests for your core changes, as applicable?

---

This change is 

[pariterre]

This fix #877 @Ipuch Please can you review and confirm that ACADOS failing is expected?

[pariterre]

This also fix a new issue in regards to numpy.testing failing after reading a biorbd.Model @Ipuch Can you confirm this fix locally on your computer as well?

[Ipuch]

@Ipuch Please can you review and confirm that ACADOS failing is expected?

Yes, it started failing from #858

This also fix a new issue in regards to numpy.testing failing after reading a biorbd.Model @Ipuch Can you confirm this fix locally on your computer as well?

It works locally just fine.