Improve test coverage

[codecov[bot]]

Codecov Report

Merging #21 (206ee46) into master (a4784e6) will increase coverage by 2.57%. The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   72.68%   75.26%   +2.57%     
==========================================
  Files          27       28       +1     
  Lines        6312     6310       -2     
==========================================
+ Hits         4588     4749     +161     
+ Misses       1724     1561     -163     

Flag	Coverage Δ
unittests	75.26% <81.25%> (+2.57%)	:arrow_up:

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Impacted Files	Coverage Δ
src/thirdparty/critic2.py	100.00% <ø> (+60.77%)	:arrow_up:
src/qtbased/guiopengl.py	50.81% <33.33%> (ø)
src/models/atomic_model.py	78.85% <72.22%> (+2.63%)	:arrow_up:
src/qtbased/mainform.py	64.94% <100.00%> (+0.35%)	:arrow_up:
src/thirdparty/crystal.py	100.00% <100.00%> (ø)
src/thirdparty/firefly.py	100.00% <100.00%> (+57.14%)	:arrow_up:
src/utils/helpers.py	100.00% <0.00%> (+2.47%)	:arrow_up:
src/utils/periodic_table.py	97.91% <0.00%> (+3.33%)	:arrow_up:
... and 2 more

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