Improve internal handling of PMFs in stan

[seabbs]

primarycensoreddist now supports efficient numerically solved correctly double censored stan PMFs (https://github.com/epinowcast/primarycensoreddist/blob/de62b580e9089a31311981ad2c1c76f477238865/inst/stan/primary_censored_dist.stan#L420). It will shortly support analytical solutions for common uses cases. See here for proposed approaches to working with stan code in primarycensoreddist: https://primarycensoreddist.epinowcast.org/articles/using-stan-tools.html