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

• Factor levels in outcome column are not preserved #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.
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