Function from dates to general linelist censored primary and secondary times

After completion of intial PR https://github.com/epinowcast/epidist/pull/125 adding as_latent_individual functionality we should continue to build out the range of data which can be processed and run with the latent_individual model. To enable date format data to be processed I propose to add a helper function mapping it to a format which can be accepted by existing as_latent_individual functionality. More specifically:

A. General format (as accepted by current as_latent_individual):

column	type
case	int
ptime_lwr	num
ptime_upr	num
stime_lwr	num
stime_upr	num
obs_at	num

B. Only dates format:

column	type
case	int
ptime_date	num or date
stime_date	num or date
obs_at	num

Proposed workflow would be:

A <- function_in_this_issue(B)
prep_obs <- as_latent_individual(A)

Maybe or out of scope for this issue

A date is just one example of a fixed censoring window. Can we have other fixed censoring windows work with this function? I think should be possible but would call this "nice-to-have"
Things to do with models outside latent_individual. Maybe this is a mistake and need to think more about how useful moving to type A will be in general?

Uncertainties

Could argue about this approach versus prep_obs <- function_in_this_issue(B) direct routes? i.e. putting the transformation inside as_latent_individual.

New issues

List more possible formats i.e. C and D
Scope out other C -> A or D -> A functions

epinowcast / epidist