add extended nll

[mmikhasenko]

Introduced wrapper of MixtureModel to hold overall normalization

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.52%. Comparing base (d305c3a) to head (a49bfdc).
⚠️ Report is 33 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #20      +/-   ##
==========================================
+ Coverage   60.27%   63.52%   +3.24%     
==========================================
  Files           6        6              
  Lines         146      159      +13     
==========================================
+ Hits           88      101      +13     
  Misses         58       58              

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