Add function that returns the prior as a dictionary

[aloctavodia]

Now we can "see" the prior being used by printing the model. But it will be nice to have direct access to the dictionary of priors. This dictionary should be the one expected by priors argument in .Model(), or a string representing it. This will make it easier to modify priors and use functions like predictive_explorer from PreliZ

[tanishy7777]

Hey I would like to work on this, could you please elaborate more about this.