Feature: Add unit tests for VintageData class

[Magnus167]

This pull request adds unit tests for the VintageData class in the simulate module. The tests cover the make_grade1, make_grade2, and make_graded methods, ensuring that the generated dataframes have the expected structure and values.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (34c182f) 81.89% compared to head (8a140f2) 83.77%.
Report is 1 commits behind head on develop.

❗ Current head 8a140f2 differs from pull request most recent head ef1c3f5. Consider uploading reports for the commit ef1c3f5 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1448      +/-   ##
===========================================
+ Coverage    81.89%   83.77%   +1.87%     
===========================================
  Files           59       59              
  Lines         6011     6011              
===========================================
+ Hits          4923     5036     +113     
+ Misses        1088      975     -113     

Files	Coverage Δ
...nergy/management/simulate/simulate_vintage_data.py	97.05% <100.00%> (+83.08%)	⬆️