Returns boolean indicating whether choice parameter supports custom value.

        If choice parameter supports custom value, user can provide parameter value
        other than those provided in choice list.

def allow_choice_custom_value(self):
        """
        Returns boolean indicating whether choice parameter supports custom value.

        If choice parameter supports custom value, user can provide parameter value
        other than those provided in choice list.
        """
        if 'choiceInfo' not in self.dto[self.name]:
            raise GPException('not a choice parameter')
        return self._is_string_true(self.dto[self.name]['choiceInfo']['choiceAllowCustom'])