Make sure `level_of_theory` is a `Level` instance if applicable

[alongd]

A followup on https://github.com/ReactionMechanismGenerator/ARC/pull/619

[codecov[bot]]

Codecov Report

Merging #620 (e649b5e) into main (d0bacdd) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #620   +/-   ##
=======================================
  Coverage   73.02%   73.03%           
=======================================
  Files          99       99           
  Lines       26175    26176    +1     
  Branches     5481     5481           
=======================================
+ Hits        19115    19118    +3     
+ Misses       5698     5697    -1     
+ Partials     1362     1361    -1     

Impacted Files	Coverage Δ
arc/main.py	48.50% <100.00%> (+0.07%)	:arrow_up:

... and 1 file with indirect coverage changes

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