Rn/improve slide examples

[rnayebi21]

Checklist

Please:

• [x ] Make sure this PR is against "dev", not "main".
• [x ] Request a review from one of the current epipredict main reviewers: dajmcdon.
• [x ] 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).
• [x ] 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).
• [x ] 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

The current examples for the sliding AR and ARX produce forecasts that could use improvement due to the complex nature of the task. Additionally, the code somewhat hard to understand, especially the k_week_ahead() function.

To address these problems, I've created a new version-faithful example that demonstrate more tractable forecasting tasks. Specifically, the example predicts the 7-day average of case rates using both lagged case rate averages and lagged death rate averages, rather than only predicting the case rate from lagged case rates. Moreover, I incorporated geo-pooling to improve the accuracy of the forecasts.

[dajmcdon]

Hey @rnayebi21 , thanks!

Before I try to review more carefully, can you:

1. merge dev into your branch and fix any conflicts? (dev now has vignette code with hard line breaks to make reviewing easier).
2. In R, run styler::style_pkg() to avoid the failing check above ☝️ . This is also available on the Addins tab at the top of RStudio.