Check that definitions file is present, and that faces file is readable.

def check_data(self):
        """Check that definitions file is present, and that faces file is readable."""
        assert os.path.exists(self.data_fp)
        if gis:
            with fiona.drivers():
                with fiona.open(self.faces_fp) as src:
                    assert src.meta

        gpkg_hash = json.load(open(self.data_fp))['metadata']['sha256']
        assert gpkg_hash == sha256(self.faces_fp)