Checking family argument

[athowes]

I think that epidist_family should pass tests of this form. It currently doesn't.

test_that("the family argument in epidist_family.epidist_latent_individual gives suitable error for non-character input", { # nolint: line_length_linter.
  epidist_family(prep_obs, family = 1)
})

I propose to add the following to epidist_formula:

1. Checking that the family passed in is a string
2. (Maybe) checking that the string is a valid PDF

I am unsure on 2.

Resources

• Use checkmate for argument checking #45

[athowes]

Created a question on Stan forum about point 2: https://discourse.mc-stan.org/t/list-of-valid-pdf-strings-for-stan/35822

[athowes]

My main point of uncertainity here whether it's worth using rstan::lookup for the price of importing rstan. Or whether we could store the list of valid LPDF functions as data and not need the package. (Then we'd need to rerun to keep it up to date.) This is for quite a small improvement in quality of life for users.