Fixed rep phi

[sbfnk]

Description

This PR closes #549.

It brings the syntax for specifying phi in line with the one for scale and allows it to be fixed.

Initial submission checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have tested my changes locally (using devtools::test() and devtools::check()).
• I have added or updated unit tests where necessary.
• I have updated the documentation if required and rebuilt docs if yes (using devtools::document()).
• I have followed the established coding standards (and checked using lintr::lint_package()).
• I have added a news item linked to this PR.

After the initial Pull Request

• I have reviewed Checks for this PR and addressed any issues as far as I am able.

[seabbs]

LGTM. News updated needed and otherwise good to go I think

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 8eb1b5d is merged into main:

• ✔️default: 31.1s -> 34.6s [-13.33%, +36.08%]
• ✔️no_delays: 40.3s -> 33.1s [-46.85%, +10.91%]
• ✔️random_walk: 9.03s -> 8.39s [-14.19%, +0.1%]
• ✔️stationary: 19.3s -> 18.7s [-21.93%, +15.59%]
• ✔️uncertain: 47.9s -> 44.7s [-27.67%, +14.06%]
  Further explanation regarding interpretation and methodology can be found in the documentation.