Convert observed data to xarray.

def observed_data_to_xarray(self):
        """Convert observed data to xarray."""
        observed_data_raw = _read_data(self.observed_data)
        variables = self.observed_data_var
        if isinstance(variables, str):
            variables = [variables]
        observed_data = {}
        for key, vals in observed_data_raw.items():
            if variables is not None and key not in variables:
                continue
            vals = np.atleast_1d(vals)
            val_dims = self.dims.get(key)
            val_dims, coords = generate_dims_coords(
                vals.shape, key, dims=val_dims, coords=self.coords
            )
            observed_data[key] = xr.DataArray(vals, dims=val_dims, coords=coords)
        return xr.Dataset(data_vars=observed_data)