refactor: simplify ungrouped epix_slide

[dshemetov]

Checklist

Please:

• Make sure this PR is against "dev", not "main" (unless this is a release
  PR).
• Request a review from one of the current main reviewers:
  brookslogan, nmdefries.
• Makes sure to bump the version number in DESCRIPTION. Always increment
  the patch version number (the third number), unless you are making a
  release PR from dev to main, in which case increment the minor version
  number (the second number).
• Describe changes made in NEWS.md, making sure breaking changes
  (backwards-incompatible changes to the documented interface) are noted.
  Collect the changes under the next release number (e.g. if you are on
  1.7.2, then write your changes under the 1.8 heading).
• See DEVELOPMENT.md for more information on the development
  process.

Change explanations for reviewer

Mostly just copied the existing epix_slide code, but removed the group_by concerns. The function is simpler and a bit more readable now.

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

• Resolves #{issue number}

[brookslogan]

Sorry, behind on PR reviews. This sounds nice for thinking about things, but scary for maintenance (code duplication & having to update multiple places). I would prefer if we maybe tagged this or, if group agrees, removed the grouping features, so we only have to update 1 epix_slide per update.

[dshemetov]

FWIW, when it comes to updating code in two or three places, I'm not that against it on principle, as long as the code comments point to similar code. But I'd also be happy to consider removing the grouped features.

[dsweber2]

Probably better to build shared functions and then call those twice (e.g. check_epix_slide_inputs eating all the things that are... checking inputs... would cover like 90% of the redundancy here with 0 code duplication).

[brookslogan]

idea: also cross-ref in epix_slide.grouped_epi_archive if not already, to remind we need to update here? Though it's easy to miss such comments in such a long function.

Sorry, I've let this PR get out of sync with slide updates, so this copy-pasta would need updated as well, rather than along the way. I was hoping we'd decide on whether to remove grouping capabilities first, but that got punted. You feel strongly that the alternative code here is worth the copy-pasta?

[dshemetov]

We'll see, maybe I'll make a wrapper like david suggests or I'll add a comment back in the grouped version, but first we'll need to rebase. I probably won't get to this PR until the end of next week, since I'm focusing on docs this week and will be on PTO Mon - Wed next week.