Feat/hist fc start stride

[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.

Summary

• Improves start handling for historical forecasts. If start is not within the trainable / forecastable points, uses the closest valid start point that is a round multiple of stride ahead of start. Raises a ValueError, if no valid start point exists.

This guarantees that all historical forecasts are n * stride points away from start, and will simplify many downstream tasks (for example conformal prediction with stridden calibration).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.88%. Comparing base (0b9efd0) to head (4395ce9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2560      +/-   ##
==========================================
- Coverage   93.88%   93.88%   -0.01%     
==========================================
  Files         139      139              
  Lines       14862    14872      +10     
==========================================
+ Hits        13953    13962       +9     
- Misses        909      910       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[madtoinou]

LGTM 🚀