Return new group_id in base64

def add_group(self, group_name, payment_address):
        """ Return new group_id in base64 """
        if (self.is_group_name_exists(group_name)):
            raise Exception("the group \"%s\" is already present"%str(group_name))
        group_id_base64 = base64.b64encode(secrets.token_bytes(32))
        self.m["groups"] += [{"group_name"      : group_name ,
                              "group_id"        : group_id_base64.decode("ascii"),
                              "payment_address" : payment_address}]
        return group_id_base64