Assume default priors for adstock_from_dict and saturation_from_dict functions

[wd60622]

These functions require the priors key in the dictionary. However, it might be convienient to assume default priors if the key is not present. This is especially useful when the user is not interested in specifying priors.

Setup:

from pymc_marketing.mmm import saturation_from_dict

Before:

saturation_from_dict({
    "lookup_name": "hill", 
    "priors": {...},
})

After

saturation_from_dict({"lookup_name": "hill"})