352 remove all instances of epi keys

[dajmcdon]

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).

Change explanations for reviewer

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

• Resolves Remove all instances of epi_keys() #352
• Possibly resolves refactor *_population_scaling() #234
• Possibly resolves Refactor step_population_scaling #346
• Possibly resolves the second point in [Perf] Bottlenecks from profiling #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

[dshemetov]

i only looked at the files you mentioned above and only had a few minor nit-type comments. overall looks good, thanks for looking into population_scaling!