feat+wip: refactor epi_archive to use S3

[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

• Attention conservation notice: probably hold off on reviewing this for now. I'm going to do a lot of clean up and testing work and then present this in one of our group meetings this week.
• The S3 implementation of epi_archive is in parallel files suffixed with _new (e.g. archive_new.R)
• All tests (that I could find) for epi_archive are in similar parallel files and they all pass
• Some S3 implementations rely on the epi_archive2 and grouped_epi_archive2 class in order to not clobber the name space or the existing code. Some other functions without S3 dispatch have parallel implementations (e.g. epix_as_of2, epix_slide2, epix_merge2, etc.)

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

• Resolves Remove R6 interface for epi_archives #340