Retain epi df 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 Steps sometimes destroy the epi_df #376 to the extent possible

[dsweber2]

How does this effect #370, deprecating the epi_recipe?

[dajmcdon]

This is a short term fix. That one is far more major (but still won't quite fix this problem). This fix is actually there as well. (Where by "fix" I mean, to the best of our ability.)

[dshemetov]

Seems like a nice-to-have partial fix.

Note: this flag activates a hook in epiprocess::as_tibble.epi_df which attempts to preserve the metadata in the returned tibble. Naturally, appending metadata to an existing class comes with no guarantees that it will be preserved or that it won't interfere with other aspects of the class, but (a) it seems unlikely that we will have issues here, (b) very very hard to understand the downstream consequences without actually trying this, so I think we should go ahead with it.

Need to fix: version number in DESCRIPTION.