Test: __str__ function in arc/statmech/arkane

[kfir4444]

As a mission to increase our coverage, testing for an untested function are added here.

[codecov[bot]]

Codecov Report

Merging #658 (5d1b9df) into main (b5a305a) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #658      +/-   ##
==========================================
+ Coverage   73.13%   73.20%   +0.06%     
==========================================
  Files          99       99              
  Lines       26289    26303      +14     
  Branches     5500     5502       +2     
==========================================
+ Hits        19227    19255      +28     
+ Misses       5699     5686      -13     
+ Partials     1363     1362       -1     

Impacted Files	Coverage Δ
arc/statmech/arkane_test.py	98.21% <100.00%> (+0.59%)	:arrow_up:

... and 2 files with indirect coverage changes

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[alongd]

Looks good! There's a second merge commit there, can you remove it?