Refactor plot_ppc_*

[ghurault]

There are four functions:

• plot_post_traj_pmf
• plot_post_traj_fanchart
• plot_ppc_traj_pmf
• plot_ppc_traj_fanchart

which are very similar as they plot posterior trajectories (post) or posterior trajectories with observed data (ppc), either as a probability mass function (pmf) or as a fanchart (fanchart).
I found it would be more elegant and straightforward if we had one function and choose the plotting style and the input would determine whether it post or ppc, or something like that.

Maybe the easiest way to do it would be using OOP, cf. #5 , so some of what is different in the functions' signature get integrated into the "object".