352 remove all instances of epi keys

[dajmcdon]

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).

Change explanations for reviewer

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

• Resolves #352
• Possibly resolves #234
• Possibly resolves #346
• Possibly resolves the second point in #372

[dajmcdon]

This PR also cleans out piles of improper prefixing (some needed, some not). The only files that need review are

1. R/key_colnames.R
2. tests/testthat/test-key_colnames.R
3. R/step_population_scaling.R
4. R/layer_population_scaling.R