Convert prior_predictive samples to xarray.

def prior_predictive_to_xarray(self):
        """Convert prior_predictive samples to xarray."""
        prior = self.prior
        prior_model = self.prior_model
        prior_predictive = self.prior_predictive
        data = get_draws_stan3(prior, model=prior_model, variables=prior_predictive)
        return dict_to_dataset(data, library=self.stan, coords=self.coords, dims=self.dims)