Refactor of epidist_formula to use S3

[athowes]

This is the only latent_individual specific part of epidist_formula.latent_individual.
Create a new internal S3 which does this and make the rest of it a regular function. as_epidist_formula, convert_formula_to_epidist_model... ask ChatGPT.

Originally posted by @athowes in #370 (comment)

In PR #338 we added double dispatch to epidist_family. I think similarly that we should refactor epidist_formula so that any non-model-specific parts are not containing within model-specific functions. I don't think we need double dispatch, just single dispatch and an outer function.

epidist_formula <- function () {
  # things here
  # including the helper in PR #370
  epidist_formula_model()
  # ...
}

Unless there are any formula specific bits? I don't think there would be.