Added a static_property to folder

[pariterre]

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This change is 

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.37%. Comparing base (6490dd5) to head (490da6a).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
bioptim/examples/utils.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1026   +/-   ##
=======================================
  Coverage   77.37%   77.37%           
=======================================
  Files         182      182           
  Lines       20186    20191    +5     
=======================================
+ Hits        15618    15623    +5     
  Misses       4568     4568           

Flag	Coverage Δ
unittests	77.37% <85.71%> (+<0.01%)	⬆️

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