Gets the SessionIndex from the AuthnStatement
        Could be used to be stored in the local session in order
        to be used in a future Logout Request that the SP could
        send to the SP, to set what specific session must be deleted

        :returns: The SessionIndex value
        :rtype: string|None

def get_session_index(self):
        """
        Gets the SessionIndex from the AuthnStatement
        Could be used to be stored in the local session in order
        to be used in a future Logout Request that the SP could
        send to the SP, to set what specific session must be deleted

        :returns: The SessionIndex value
        :rtype: string|None
        """
        session_index = None
        authn_statement_nodes = self.__query_assertion('/saml:AuthnStatement[@SessionIndex]')
        if authn_statement_nodes:
            session_index = authn_statement_nodes[0].get('SessionIndex')
        return session_index