scoot docstrings in optimization_variable.py

[rapidnico]

This change is 

[codecov]

Codecov Report

Base: 83.67% // Head: 83.67% // No change to project coverage 👍

Coverage data is based on head (7297319) compared to base (c140068).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #532   +/-   ##
=======================================
  Coverage   83.67%   83.67%           
=======================================
  Files          89       89           
  Lines        9909     9909           
=======================================
  Hits         8291     8291           
  Misses       1618     1618           

Impacted Files	Coverage Δ
bioptim/optimization/optimization_variable.py	85.29% <ø> (ø)

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[pariterre]

@rapidnico
Thanks for starting contributing to bioptim!
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