Convert prior samples to xarray.

def prior_to_xarray(self):
        """Convert prior samples to xarray."""
        prior = self.prior
        # filter posterior_predictive and log_likelihood
        prior_predictive = self.prior_predictive
        if prior_predictive is None:
            prior_predictive = []
        elif isinstance(prior_predictive, str):
            prior_predictive = [prior_predictive]

        ignore = prior_predictive + ["lp__"]

        data = get_draws(prior, ignore=ignore)
        return dict_to_dataset(data, library=self.pystan, coords=self.coords, dims=self.dims)