Convert prior_predictive samples to xarray.

def prior_predictive_to_xarray(self):
        """Convert prior_predictive samples to xarray."""
        prior = self.prior
        prior_predictive = self.prior_predictive
        data = get_draws(prior, variables=prior_predictive)
        return dict_to_dataset(data, library=self.pystan, coords=self.coords, dims=self.dims)