Adding constitutive model as necessary input to formulation modify_fi…

[chamel3]

…eld_gradient method so we can implement things like PlaneStress

[codecov]

Codecov Report

❌ Patch coverage is 55.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.01%. Comparing base (5f63456) to head (386b116).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
pancax/physics_kernels/base.py	42.85%	8 Missing ⚠️
pancax/post_processor.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
- Coverage   70.13%   70.01%   -0.12%     
==========================================
  Files          91       91              
  Lines        4135     4142       +7     
==========================================
  Hits         2900     2900              
- Misses       1235     1242       +7     

Flag	Coverage Δ
unittests	70.01% <55.00%> (-0.12%)	⬇️

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