public void addFeature(String key, String value) {
        addFeature(key, value, ALL_SUB_QUERIES);
    }
Adds a feature/value-pair to the predicate query. This feature is applied to all sub queries.

@param key name of the feature to be set in this query
@param value value of the feature
container-search/src/main/java/com/yahoo/prelude/query/PredicateQueryItem.java