Fix missing normalization factor in likelihood

[vhaasteren]

This adds a normalization factor in the likelihood so that the likelihood is normalized.

This fixes #405

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.74%. Comparing base (6fa7e04) to head (d983de7).
Report is 8 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #406      +/-   ##
==========================================
+ Coverage   71.64%   71.74%   +0.09%     
==========================================
  Files          13       13              
  Lines        3231     3228       -3     
==========================================
+ Hits         2315     2316       +1     
+ Misses        916      912       -4     

Files with missing lines	Coverage Δ
enterprise/signals/signal_base.py	90.23% <100.00%> (+0.02%)	⬆️

... and 3 files with indirect coverage changes

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