adding implementation of plane stress. Also adding some unit testing …

[chamel3]

…for different mechanics formulations.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.29%. Comparing base (386b116) to head (0e9edb1).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
+ Coverage   70.01%   70.29%   +0.27%     
==========================================
  Files          91       91              
  Lines        4142     4154      +12     
==========================================
+ Hits         2900     2920      +20     
+ Misses       1242     1234       -8     

Flag	Coverage Δ
unittests	70.29% <100.00%> (+0.27%)	⬆️

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