Add add_id function

[MemoOlv]

Adding function to include id from a dataframe.

• ⬆️ Adding add_id function into dummy_model
• ✅ Add test_add_id function
• 🩹 Give format to train_power_law_model function

[codecov[bot]]

Codecov Report

Merging #27 (dae74ba) into develop (3459113) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop      #27   +/-   ##
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  Coverage    95.23%   95.23%           
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  Files            2        2           
  Lines           21       21           
========================================
  Hits            20       20           
  Misses           1        1           

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