fix index generation and n steps between

[dennisbader]

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 #2352.

Summary

• fixes a bug where n_steps_between did not work properly with custom business frequencies
• fixes a bug where generate_index() with a negative frequencies and either start or end not intersecting with the frequency, did not return the correct index.
• fixes other edge cases with generate_index() and n_steps_between

[codecov]

Codecov Report

Attention: Patch coverage is 97.14286% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 94.02%. Comparing base (ca6a630) to head (9a1beeb).
Report is 1 commits behind head on master.

❗ Current head 9a1beeb differs from pull request most recent head 93d2b9a. Consider uploading reports for the commit 93d2b9a to get more accurate results

Files	Patch %	Lines
darts/utils/utils.py	97.05%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2357      +/-   ##
==========================================
+ Coverage   94.00%   94.02%   +0.02%     
==========================================
  Files         138      138              
  Lines       14201    14216      +15     
==========================================
+ Hits        13350    13367      +17     
+ Misses        851      849       -2     

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