extend(items) {
        if (items instanceof StoppingCriteriaList) {
            items = items.criteria;
        } else if (items instanceof StoppingCriteria) {
            items = [items];
        }
        this.criteria.push(...items);
    }
Adds multiple stopping criteria to the list.

@param {StoppingCriteria|StoppingCriteriaList|StoppingCriteria[]} items The stopping criteria to add.