Issue #266: Include intercept when no model provided

[athowes]

Description

This PR closes #266.

It:

• Adds a utility function .make_intercepts_explicit which takes a formula and puts a model dpar ~ 1 on any distributional parameters which 1. are not fixed, 2. are not mu, 3. do not have a formula
• This utility function is used in epidist_formula.epidist_latent_individual
  • Note that with Refactor of epidist_formula to use S3 #372 we should move this out of being model-specific
• Simplifies epidist_family_prior.lognormal because no longer have to set priors on the untransformed sigma
• Requires follow up work to:
  • Simplify epidist_family_prior.lognormal with restructure. Probably this fits into Generalise handling priors for parameters set to constant #306
  • Add new issue on shifting to shifting to brms style formula input rather than require brms::bf() as discussed in No model meta #307
• Also fixed a bug in test that brms samples from the prior right which was highlighted by these changes 9d19d49 (not sure how it was passing before, perhaps because there wasn't an explicit model it worked OK)

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title in the for Issue(s) issue-numbers: PR title
• I have read the contribution guidelines.
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• My code follows the established coding standards.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[seabbs]

Nice this is great. All follow up tasks look good to me.