seperate material + stress iteration caches

[kimauth]

Counter suggestion for #42

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.95652% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.37%. Comparing base (4d791e9) to head (183eda5).
⚠️ Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
src/iteration_caches.jl	76.92%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
- Coverage   92.12%   91.37%   -0.76%     
==========================================
  Files          12       13       +1     
  Lines         432      452      +20     
==========================================
+ Hits          398      413      +15     
- Misses         34       39       +5     

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[lijas]

I dont really see the difference with this PR and #42. It seems like CacheContainer is just an extra layer, since it is only used when plane stress is needed? So the resulting solver_cache() and plane_stress_cache() just becomes extra boilerplate. Also, the material_response(::Plastic...) seems to also get the PlateStress cache, which it will never need.

Tbh, I think it is okay if we skip making the plane stress iterations allocation free, since it seems to complicate the code to much :P