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

[EveCharbie]

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? issues Should external forces be numerical values or states ? #657
• 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?

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This change is 

[pariterre]

Reviewed 11 of 11 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @EveCharbie)

[pariterre]

Reviewed 13 of 13 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @EveCharbie)

[codecov]

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 ⚠️
bioptim/models/biorbd/biorbd_model.py	95.00%	1 Missing ⚠️

Additional details and impacted files

@@           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	Coverage Δ
unittests	78.51% <94.44%> (+<0.01%)	⬆️

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[EveCharbie]

@pariterre can be merged

[pariterre]

Reviewed 1 of 1 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @EveCharbie)