Document epidist_latent_individual class specific functions

[athowes]

The epidist_latent_individual class functions are currently not well documented.

I do not have a perfect picture of how to handle documenting instances of methods. (Also what the name is for functions like this e.g. epidist_prepare.epidist_latent_individual.)

As a part of 1. finding out best practise here 2. implementing then I'd also want the roxygen2 website to display nicely (e.g. grouping together class specific functions?).

See #70.

[seabbs]

you can have a look at scoringutils here for one approach and there is a issue arguing about this (will look for)

[athowes]

I think the issue arguing about this might be epiforecasts/scoringutils#793

[athowes]

Closing as outdated / non-actionable.