Improve LOC to accommodate Gd2O3, use burnup units

[sskutnik]

Minor feature enhancements to low-order consistency checking:

• Automatically tally initial Gd2O3 from initial state and include as mass fraction in `lumped0d-uox.jt.inp.
• Use burnup units on LOC isotope normalized error plotting

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.78%. Comparing base (1fcdf19) to head (0078754).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   82.75%   82.78%   +0.03%     
==========================================
  Files          16       16              
  Lines        3763     3770       +7     
==========================================
+ Hits         3114     3121       +7     
  Misses        649      649              

Flag	Coverage Δ
unittests	82.78% <100.00%> (+0.03%)	⬆️

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