fit() no longer drops the epi_workflow class

[dajmcdon]

Checklist

Please:

• Make sure this PR is against "dev", not "main".
• Make sure to bump the version number in DESCRIPTION and NEWS.md.
  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
  0.7.2, then write your changes under the 0.8 heading).

Change explanations for reviewer

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

• Resolves fit.epi_workflow silently drops epi_workflow class when built or fit on a non-epi_df or non-epi_recipe in most cases #363

[dshemetov]

Seems fine to me, modulo a small non-blocking suggestion to error when epi_recipe is given non-epi_df data instead of decaying to recipe.

Need to fix: version number in DESCRIPTION.