Fix/inverse transform for static covariate with single category across series

[madtoinou]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Fixes #2705.

Summary

• Each covariates is referred to by the combination of covariates name and category name, regardless of the number of categories (used to be for covariates with more than 1 category).

Other Information

This might be a breaking changes for some as it can change the number of covariates in the series but very unlikely in my opinion.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.10%. Comparing base (62dca8f) to head (7130b72).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2710      +/-   ##
==========================================
- Coverage   94.17%   94.10%   -0.08%     
==========================================
  Files         141      141              
  Lines       15596    15591       -5     
==========================================
- Hits        14688    14672      -16     
- Misses        908      919      +11     

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[dennisbader]

Very nice fix, thanks a lot @madtoinou 🚀 🚀 🚀