Add reference_values option to ppc plot

[aloctavodia]

In some functions in PreliZ we allow the user to pass reference values. scalars lists/tuples or dictionaries are allowed, the dictionaries are used to pass labels and values. This can be particularly useful for prior predictive checks, like in the next figure.

[imperorrp]

This can be done, we could pass this to plot_collection.map() and apply a rotation to the annotate-xy visual element text for the vertical annotation