Create an EczemaFit class inheriting from stanfit

[ghurault]

The idea is that, instead of EczemaFit outputting a stanfit object, we could create a class "EczemaFit" that inherits from stanfit, so that we use standard functions (e.g. from rstanarm, bayesplot or base) such as plot instead of plot_latent_OrderedRW and plot_transition_MC; and pp_check (with plotfun argument) instead of plot_ppc_traj_* and plot_post_traj_*.

For instance, the EczemaFit object could contain an EczemaModel object, the data (training and testing) and the Stanfit object.