@Override
        public void order(List<Hit> hits) {
            // The order of hits is given by the grouping framework, and should not be re-ordered when we copy the hits
            // from the groups to the base HitGroup in the result.
        }
Implements 'unique' using a grouping expression.

It doesn't work for multi-level sorting.

@author andreer
container-search/src/main/java/com/yahoo/search/grouping/UniqueGroupingSearcher.java