Extracts data from mbox files. Mutates _data.

        :param str source: The path to one or more mbox files.
        :param args: Arbitrary arguments for extensibility.
        :param kwargs: Arbitrary keyword arguments for extensibility.
        :return: None

def _extract(self, source, *args, **kwargs):
        """
        Extracts data from mbox files. Mutates _data.

        :param str source: The path to one or more mbox files.
        :param args: Arbitrary arguments for extensibility.
        :param kwargs: Arbitrary keyword arguments for extensibility.
        :return: None
        """
        # Extract data
        self._data = mbox_to_pandas(source)
        self._data['MessageID'] = pd.Series(range(0,len(self._data)))