Gets the NameID Format of the Logout Request Message
        :param request: Logout Request Message
        :type request: string|DOMDocument
        :param key: The SP key
        :type key: string
        :return: Name ID Format
        :rtype: string

def get_nameid_format(request, key=None):
        """
        Gets the NameID Format of the Logout Request Message
        :param request: Logout Request Message
        :type request: string|DOMDocument
        :param key: The SP key
        :type key: string
        :return: Name ID Format
        :rtype: string
        """
        name_id_format = None
        name_id_data = OneLogin_Saml2_Logout_Request.get_nameid_data(request, key)
        if name_id_data and 'Format' in name_id_data.keys():
            name_id_format = name_id_data['Format']
        return name_id_format