Minor refactor of z-score to use the correct standard deviation

[myrmarachne]

[codecov-commenter]

Codecov Report

Merging #136 (ee96a43) into master (5fc5ed9) will increase coverage by 0.09%. The diff coverage is n/a.

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@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
+ Coverage   76.71%   76.81%   +0.09%     
==========================================
  Files         227      227              
  Lines        9071     9071              
==========================================
+ Hits         6959     6968       +9     
+ Misses       2112     2103       -9     

see 1 file with indirect coverage changes

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