[RTM] external forces as symbolic variables to allow multi-threading

[EveCharbie]

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? issues #657
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This change is 

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.44444% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 78.51%. Comparing base (4cda59c) to head (4cc49ef). Report is 55 commits behind head on master.

Files	Patch %	Lines
bioptim/dynamics/configure_problem.py	75.00%	2 Missing :warning:
bioptim/models/biorbd/biorbd_model.py	95.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #865 +/- ## ======================================= Coverage 78.50% 78.51% ======================================= Files 141 141 Lines 16309 16296 -13 ======================================= - Hits 12804 12794 -10 + Misses 3505 3502 -3 ``` | [Flag](https://app.codecov.io/gh/pyomeca/bioptim/pull/865/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/865/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pyomeca) | `78.51% <94.44%> (+<0.01%)` | :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.

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[EveCharbie]

@pariterre can be merged