def cogroup(*others)
      unioned = self
      others.each do |other|
        unioned = unioned.union(other)
      end

      unioned.group_by_key
    end
def cogroup(*others)
      unioned = self
      others.each do |other|
        unioned = unioned.union(other)
      end

      unioned.group_by_key
    end
For each key k in `this` or `other`, return a resulting RDD that contains a tuple with the
 list of values for that key in `this` as well as `other`.

 == Example:
   rdd1 = $sc.parallelize([["a", 1], ["a", 2], ["b", 3]])
   rdd2 = $sc.parallelize([["a", 4], ["a", 5], ["b", 6]])
   rdd3 = $sc.parallelize([["a", 7], ["a", 8], ["b", 9]])
   rdd1.cogroup(rdd2, rdd3).collect
   # => [["a", [1, 2, 4, 5, 7, 8]], ["b", [3, 6, 9]]]