Issue #386: Add direct model allowing pass through to brms

[athowes]

Description

This PR closes #386.

It adds a pass through model called direct_model and the infrastructure needed to support it. It also adds tests for that model (both unit and integration).

Remaining things to discuss IMO:

• In the class constructor for direct_model I call the times ptime and stime -- this seems debateable: Update pass through model constructor to use epidist_linelist #402
• Consider name different to direct_model: Rename latent_individual and direct_model for consistency #400
• Follow up issue on using this in the getting started vignette to show why our models (e.g. latent_individual are better): Use naive model in getting started vignette to highlight impact of not adjusting #403

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.
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[seabbs]

Before I review can you follow up on where you have landed with "Remaining things to discuss IMO" points?

[athowes]

Before I review can you follow up on where you have landed with "Remaining things to discuss IMO" points?

1. In the class constructor for direct_model I call the times ptime and stime -- this seems debateable: I think this is something we can figure out during the proprocessing work. As it sort of relates to e.g. taking epidist_linelist as input here

2. Consider name different to direct_model: I'm fine with direct_model. One aspect that is dislikeable about it is that it ends in model. Whereas latent_individual doesn't. So maybe it should just be direct. There are lots of other possible options but I'm interested to hear your POV

3. Follow up issue on using this in the getting started vignette to show why our models (e.g. latent_individual are better): yes this is something I can make

[seabbs]

Can you give a ping when this is ready for re-review as the tracker has got a bit confused

[athowes]

Can you give a ping when this is ready for re-review as the tracker has got a bit confused

Ready now. Should have things merged in, and added issues for all follow-up.