Pi-stacking fix

[cbouy]

Fixes #68, #76

Changes the implementation for pi-stacking detection to a more accurate one.

[codecov]

Codecov Report

Merging #97 (0918f7b) into master (ad51a73) will decrease coverage by 0.67%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   95.58%   94.90%   -0.68%     
==========================================
  Files           9        9              
  Lines         997     1041      +44     
==========================================
+ Hits          953      988      +35     
- Misses         44       53       +9     

Impacted Files	Coverage Δ
prolif/fingerprint.py	96.41% <ø> (ø)
prolif/utils.py	99.03% <80.00%> (-0.97%)	⬇️
prolif/interactions.py	99.20% <98.24%> (-0.32%)	⬇️
prolif/plotting/network.py	92.57% <0.00%> (-3.06%)	⬇️

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