Add the choice for an integrator of the solution

[Ipuch]

Related to #401

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?

[pariterre]

GG so far :)

[pariterre]

Same comment :P

[pariterre]

LGTM

[codecov]

Codecov Report

Merging #405 (e16441c) into master (ef7c099) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
+ Coverage   80.59%   80.61%   +0.01%     
==========================================
  Files          81       81              
  Lines        8374     8381       +7     
==========================================
+ Hits         6749     6756       +7     
  Misses       1625     1625              

Impacted Files	Coverage Δ
bioptim/__init__.py	100.00% <100.00%> (ø)
bioptim/gui/plot.py	76.36% <100.00%> (ø)
bioptim/misc/enums.py	100.00% <100.00%> (ø)
bioptim/optimization/optimal_control_program.py	88.34% <100.00%> (ø)
bioptim/optimization/solution.py	85.18% <100.00%> (ø)

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