Unifying stan files for models with and without seroreversion

[ntorresd]

We want to reduce the number of stan files in the package (./inst/stan/*.stan). This will facilitate adding additional models (like IID models) and (maybe) reduce compilation time of the package during installation.

The main differences between the models with and without seroreversion are:

• Declaration of the data in data/seroreversion_prior_data.stan

  // prior index
  int seroreversion_prior_index;
  // seroreversion prior choices
  real<lower=0> seroreversion_min;
  real<lower=seroreversion_min> seroreversion_max;
  // normal
  real<lower=0> seroreversion_mean;
  real<lower=0> seroreversion_sd;

• Declaration of the parameter seroreversion_rate in the parameters chunk:

parameters {
  ...
  real<lower=0> seroreversion_rate;
  ...
}

• Evaluation of functions prob_infected_* seroreversion parameter, i.e. seroreversion_rate = 0 when is_seroreversion = FALSE in fit_seromodel()