Convert posterior_predictive samples to xarray.

def posterior_predictive_to_xarray(self):
        """Convert posterior_predictive samples to xarray."""
        data = self.posterior_predictive
        if not isinstance(data, dict):
            raise TypeError("DictConverter.posterior_predictive is not a dictionary")

        return dict_to_dataset(data, library=None, coords=self.coords, dims=self.dims)