Fixes #278: LoG scale normalization

[gschivre]

Simply replace -σ by -σ^2 in multiLoG and update the docstring of BlobLoG.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.81%. Comparing base (3b3d88a) to head (c6e854c).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #279   +/-   ##
=======================================
  Coverage   91.81%   91.81%           
=======================================
  Files          12       12           
  Lines        1662     1662           
=======================================
  Hits         1526     1526           
  Misses        136      136           

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[ViralBShah]

Would be nice for someone else in the project to chime in to get this merged.