Add test for converting full models

[paulromano]

This PR adds a new tests/models/ directory where we can put full MCNP models and test conversion. Right now, there is an associated test which simply runs mcnp_to_openmc on the model and makes sure it completes successfully. In the future, we can expand to do more elaborate checks.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.90%. Comparing base (e2d6fcf) to head (a8b1c03).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
+ Coverage   72.51%   81.90%   +9.39%     
==========================================
  Files           3        3              
  Lines         724      724              
  Branches      190      190              
==========================================
+ Hits          525      593      +68     
+ Misses        149       73      -76     
- Partials       50       58       +8     

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