public HitValue subsetFilter(Function<Hashlet<String,Integer>, Hashlet<String,Integer>> filter) {
            return new HitValue(filter.apply(hashlet), dataValues, doubleValues);
        }
MatchFeatureData helps pack match features for hits into
inspectable HitValue objects, all sharing the same Hashlet
for the field names.

@author arnej
vespajlib/src/main/java/com/yahoo/data/access/helpers/MatchFeatureData.java