Convert the posterior to an xarray dataset.

def posterior_to_xarray(self):
        """Convert the posterior to an xarray dataset."""
        data = {}
        for i, var_name in enumerate(self.var_names):
            data[var_name] = np.expand_dims(self.posterior[i], axis=0)
        return dict_to_dataset(data, library=self.tfp, coords=self.coords, dims=self.dims)