Convert posterior_predictive samples to xarray.

def posterior_predictive_to_xarray(self):
        """Convert posterior_predictive samples to xarray."""
        data = {k: np.expand_dims(v, 0) for k, v in self.posterior_predictive.items()}
        return dict_to_dataset(data, library=self.pymc3, coords=self.coords, dims=self.dims)