Add entity state validator
        :param validator: a validator, implementing AbstractValidator
        :return: None

def add_state_validator(self, validator):
        """
        Add entity state validator
        :param validator: a validator, implementing AbstractValidator
        :return: None
        """
        if not isinstance(validator, AbstractValidator):
            err = '{} is not a subclass of {}'
            raise InvalidValidator(err.format(validator, AbstractValidator))

        if validator not in self.state:
            self.state.append(validator)