Adjustement to examples using the good force parameter

[Dani-dB]

Before, when giving the parameter to the overdamped class in :
model_simu = fl.models.overdamped.Overdamped(force_function, diffusion=diff_function) we had :
force_function = $- \dfrac{\partial U(q)}{\partial q}$ .
Now it is the appropriate
force_function = $- D(q)\dfrac{\partial U(q)}{\partial q}$
Moreover, I corrected the formula to recover the free energy starting from the Milstein integrator.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 28.70%. Comparing base (d90489c) to head (a5061ec).
Report is 2 commits behind head on main.

Files	Patch %	Lines
folie/analysis/overdamped_1d.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #12       +/-   ##
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- Coverage   72.66%   28.70%   -43.97%     
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  Files          29       29               
  Lines        2133     2139        +6     
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- Hits         1550      614      -936     
- Misses        583     1525      +942     

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