Model.prior_predictive() does not check if the model was built

[tomicapretto]

If you build the model using a certain set of priors, and then update the priors, Model.prior_predictive() will work with the first set of priors, ignoring the updated version. This is because the underlying PyMC model still keeps the original priors. I think prior_predictive() should check whether Model.built is True as it happens with Model.plot_priors().