Fixes for 2d struct simulation

[ktbolt]

I've fixed some indexing and an Eigen error in material models (#557).

[ktbolt]

I will add a test for this.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.07%. Comparing base (f521770) to head (4bae8b0).

Files with missing lines	Patch %	Lines
Code/Source/solver/read_files.cpp	50.00%	4 Missing ⚠️
Code/Source/solver/mat_fun.h	57.14%	3 Missing ⚠️
Code/Source/solver/mat_models.cpp	50.00%	2 Missing ⚠️
Code/Source/solver/sv_struct.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #562      +/-   ##
==========================================
+ Coverage   69.04%   69.07%   +0.02%     
==========================================
  Files         181      181              
  Lines       34121    34132      +11     
  Branches     5897     5901       +4     
==========================================
+ Hits        23560    23577      +17     
+ Misses      10424    10417       -7     
- Partials      137      138       +1     

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[hanzhao2020]

@ktbolt Looks good to me, thanks for making these changes!

[ktbolt]

@divya-adil Please review.