Feedback fix daria

[dariakhv]

addressed peer review feedback:

1. DongchunChen: predict_sales could include additional metrics, the function now only prints MSE --> predict_sales.py
2. Jenson-Chen: test_predict_sales.py imports test data while the other two tests uses pytest_fixture. --> test_predict_sales.py + removed test_data.csv
3. Lukman-Lateef include a general interpretation of MSE and the prediction result in the vignette --> example.ipynb

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.10%. Comparing base (7d8168a) to head (f8b4fab).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #56      +/-   ##
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+ Coverage   99.09%   99.10%   +0.01%     
==========================================
  Files           4        4              
  Lines         110      112       +2     
==========================================
+ Hits          109      111       +2     
  Misses          1        1              

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