Fix flake8 problems

[codecov[bot]]

Codecov Report

Merging #17 (571e7af) into master (42b5aaa) will increase coverage by 0.15%. The diff coverage is 73.33%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
+ Coverage   64.76%   64.92%   +0.15%     
==========================================
  Files          24       24              
  Lines        5889     5907      +18     
==========================================
+ Hits         3814     3835      +21     
+ Misses       2075     2072       -3     

Flag	Coverage Δ
unittests	64.92% <73.33%> (+0.15%)	:arrow_up:

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Impacted Files	Coverage Δ
src/models/graphene.py	100.00% <ø> (ø)
src/models/swnt.py	100.00% <ø> (ø)
src/models/volumericdata.py	32.69% <0.00%> (ø)
src/qtbased/pyqtgraphwidget.py	100.00% <ø> (ø)
src/qtbased/pyqtgraphwidgetimage.py	100.00% <ø> (ø)
src/utils/helpers.py	97.36% <ø> (-0.05%)	:arrow_down:
src/utils/calculators.py	79.60% <5.00%> (ø)
src/qtbased/atomidentifier.py	20.93% <40.00%> (ø)
src/qtbased/mainform.py	45.03% <55.13%> (ø)
src/qtbased/guiopengl.py	36.58% <77.77%> (+0.06%)	:arrow_up:
... and 9 more

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