Extract sample_stats_prior from prior.

def sample_stats_prior_to_xarray(self):
        """Extract sample_stats_prior from prior."""
        prior = self.prior
        prior_model = self.prior_model
        data = get_sample_stats_stan3(prior, model=prior_model)
        return dict_to_dataset(data, library=self.stan, coords=self.coords, dims=self.dims)