slide_mean and slide_sum

[dsweber2]

Checklist

Please:

• Make sure this PR is against "dev", not "main".
• Request a review from one of the current epipredict main reviewers:
  dajmcdon.
• 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).
• Consider pinning the epiprocess version in the DESCRIPTION file if
  • You anticipate breaking changes in epiprocess soon
  • You want to co-develop features in epipredict and epiprocess

Change explanations for reviewer

This is a continuation of #275 that adds the specific methods for step_epi_slide_mean and step_epi_slide_sum; step_epi_slide_opt would be similar, though hasn't yet been implemented. step_epi_slide should have a warning/dispatch to these methods, though at the moment step_epi_slide doesn't NA pad, while these do. Currently leaving this partially finished until we need the comparatively faster speeds of these methods.

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

• Resolves #{issue number}