Hotfix growth rate

[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).
• 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 changes is necessary for CI which builds from epiprocess >= 0.10.4. (following No genlasso epiprocess#595)
• Note also the minor related issue growth_rate() gives hard error in corner cases epiprocess#605

[dsweber2]

I was just running into this over on #431; I'm surprised the tests for step_growth_rate were passing before explicitly specifying which argument is x and which is y for growth_rate.

[brookslogan]

Looks like it should work. I'm getting test failures locally from test-bake-method.R and test-population_scaling.R but they appear to be unrelated.