Allow the user to not expand the dynamics if wanted

[Ipuch]

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This PR is merely intended for custom dynamics.

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This change is 

[codecov]

Codecov Report

Merging #454 (4aa35f7) into master (0a86e9a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #454   +/-   ##
=======================================
  Coverage   80.48%   80.48%           
=======================================
  Files          85       85           
  Lines        8996     8998    +2     
=======================================
+ Hits         7240     7242    +2     
  Misses       1756     1756           

Impacted Files	Coverage Δ
bioptim/dynamics/configure_problem.py	91.30% <100.00%> (+0.05%)	⬆️

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[pariterre]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Ipuch)

[pariterre]

That is PR as I like them!
LGTM :)