Preserve factor levels in outcome column

[kelly-sovacool]

Design discussion w/ @courtneyarmour:

• this only matters when outcome is binary.
• force users to supply a factor for outcome column when outcome is binary -- throw an error otherwise.
• don't use pos_outcome argument - feature bloat.
• make sure original factor levels preserved in calc perf metrics.

Issues

• #339

Change(s) made

Checklist

(~Strikethrough~ any points that are not applicable.)

• [ ] Write unit tests for any new functionality or bug fixes.
• [ ] Update docs if there are any API changes:
  • [ ] roxygen comments
  • [ ] vignettes
• [ ] Update NEWS.md if this includes any user-facing changes.
• [ ] The check workflow succeeds on your most recent commit. This is always required before the PR can be merged.