Issue #372: Refactor formula dispatch

[athowes]

Description

This draft PR will close #372.

I am considering doing #382 as well since #372 involves reorganising the function locations. In doing so it seems easy to also fix the other function locations. I'd like to move quickly.

What has been done:

On #372:

• epidist_formula is now a regular function which contains all processing that happens across models
• It calls epidist_formula_model which dispatches on the class of data into the model specific part
• Moved into new file formula.R and the tests are moved into a test-formula.R file

On #382:

• Moves formula functions into formula.R

Edit: I'm going to let this get to passing tests and if it's quick and I can catch a reviewer I'll put the #382 into a new PR.

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]

Really nice. One small comment about opening up the default method now that this is optional.

[athowes]

Actually we have the same issue in family:

#' Default method for defining a model specific family
#'
#' @inheritParams epidist_family_model
#' @param ... Additional arguments passed to method.
#' @family family
#' @export
epidist_family_model.default <- function(data, ...) {
  cli_abort(
    "No epidist_family_model method implemented for the class ", class(data),
    "\n", "See methods(epidist_family_model) for available methods"
  )
}

Can fix both here.

Edit: confirming that in epidist we call epidist_validate which already does this check, so for sure we don't need it.

[seabbs]

Nice. LGTM