Convert posterior_predictive samples to xarray.

def posterior_predictive_to_xarray(self):
        """Convert posterior_predictive samples to xarray."""
        posterior = self.posterior
        posterior_predictive = self.posterior_predictive
        data = get_draws(posterior, variables=posterior_predictive)
        return dict_to_dataset(data, library=self.pystan, coords=self.coords, dims=self.dims)