public Grouping setLastLevel(int level) {
        lastLevel = level;
        return this;
    }
Sets the last level to do grouping work. Executing a level will instantiate the {@link Group} objects for the
next level, if there is any. This means that grouping work ends at this level, but also instantiates the groups
for level (lastLevel + 1).

@param level the level to set
@return this, to allow chaining
searchlib/src/main/java/com/yahoo/searchlib/aggregation/Grouping.java