feat: introduce epix_as_of_current() for convenience

[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

REVISED:

• Add epix_as_of_current(), which is an alias for epix_as_of(.$versions_end)
  • Separate function because epix_as_of() reads poorly
• Added set_versions_end() to bypass some epix_as_of() limitations
• Fixed a bug with yearmonth in epix_as_of and epix_slide

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

• Resolves [discussion] epix_as_of(as_of) default setting #608
• Resolves epix_slide fails on yearmonths #648

[brookslogan]

Did a skim. I should probably look more carefully + actually read the tests; going to stop here & self-re-review-request.

[brookslogan]

Looks good and nearly complete.

• But @dsweber2's question/comment today makes me think we need to either:
  • Go ahead and place a check here that "now" isn't too far from the pre-existing version end and/or max.
  • Not do the current-time/version stuff and stick with as-of versions_end.

Otherwise we are going to potentially fool people who are attempting to be careful by checking the as_of of the epi_df we give them, by giving them an as_of that wasn't actually recorded and may be nowhere near to one that was recorded.

[Probably I could be convinced otherwise if that's what would actually be helpful in real production forecasting. Just marking this as "Comment" so someone else can go ahead & Approve if that's the case or just bypass & merge.]

[dshemetov]

Switching back to epix_as_of_current() and rebasing.

[dsweber2]

version needs to be bumped, but on a quick read-through looks good