docs: public static Map<String,List<Writable>> getUnique(List<String> columnNames, Schema schema, JavaRDD<List<Writable>> data){
          Map<String,Set<Writable>> m = data.aggregate(null, new UniqueAddFunction(columnNames, schema), new UniqueMergeFunction());
          Map<String,List<Writable>> out = new HashMap<>();
          for(String s : m.keySet()){
              out.put(s, new ArrayList<>(m.get(s)));
          }
          return out;
      }