fix(vaep): use iloc to select last value in the dataframe

[abhishekvasu94]

This commit fixes a bug in the scores and concedes functions in vaep, where the intention is to grab the last element in the dataframe and assign that value to the end of the shifted dataframe

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.84%. Comparing base (5f52604) to head (b1bd166).
Report is 62 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #718      +/-   ##
==========================================
+ Coverage   83.24%   83.84%   +0.59%     
==========================================
  Files          48       48              
  Lines        3677     3707      +30     
  Branches      625      629       +4     
==========================================
+ Hits         3061     3108      +47     
+ Misses        507      489      -18     
- Partials      109      110       +1     

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