[RTM when modify] spring load - explore more objective function - tests will come later

[mickaelbegon]

All Submissions:

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

New Feature Submissions:

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

Changes to Core Features:

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

---

This change is 

[codecov[bot]]

Codecov Report

Attention: 22 lines in your changes are missing coverage. Please review.

Comparison is base (53e60c8) 78.72% compared to head (d2e6dad) 78.90%. Report is 7 commits behind head on master.

Files	Patch %	Lines
bioptim/examples/torque_driven_ocp/spring_load.py	37.14%	22 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #840 +/- ## ========================================== + Coverage 78.72% 78.90% +0.17% ========================================== Files 140 139 -1 Lines 15742 15721 -21 ========================================== + Hits 12393 12404 +11 + Misses 3349 3317 -32 ``` | [Flag](https://app.codecov.io/gh/pyomeca/bioptim/pull/840/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pyomeca) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pyomeca/bioptim/pull/840/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pyomeca) | `78.90% <47.61%> (+0.17%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pyomeca#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.