Fix `Scenario` errors in `add_year()`

[glatterf42]

Closes #578. In that issue, @gidden mentioned that this branch by Behnam contained a fix for the issue. However, that branch has become severely outdated over the years. So after rebasing #494, I created a new branch here that captures the changes contained in @behnam-zakeri's branch.

I'm not sure what's still missing in #494, but I think this could be a good opportunity to move things along. And the assessments in #578 made me think that things need to move along to address these known bugs.

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• [ ] Continuous integration checks all ✅
• [ ] Add or expand tests; coverage checks both ✅
• [ ] Add, expand, or update documentation.
• [ ] Update release notes.