Add a keyword.

        Args:
            keyword(str): keyword to add.
            schema(str): schema to which the keyword belongs.
            source(str): source for the keyword.

def add_keyword(self, keyword, schema=None, source=None):
        """Add a keyword.

        Args:
            keyword(str): keyword to add.
            schema(str): schema to which the keyword belongs.
            source(str): source for the keyword.
        """
        keyword_dict = self._sourced_dict(source, value=keyword)

        if schema is not None:
            keyword_dict['schema'] = schema

        self._append_to('keywords', keyword_dict)