use rstan::gqs() to run posterior predictions of KM script

[n8thangreen]

create a stan_model of postpred_.stan first and then run with prerun output of posterior samples

postpred_exp_exp_gqs.stan isnt working at the moment because of some weird error...

https://mc-stan.org/rstan/reference/stan_model.html
https://mc-stan.org/rstan/reference/stanmodel-method-gqs.html

[n8thangreen]

Written R script to do this outside of Stan
d669b7b