Uakyol/stratified sketch noise

[uakyol]

Here is the second part of the stratified sketch where we introduce noising. We introduce a epsilon_split parameter to split the privacy budget into two parts: For 1+ sketch and other sketches.

   We support creation of 1+ sketch for 2 scenerios :
     1) 1+ sketch is created from the underlying exact set directly. Here we
     noise 1+ sketch with epsilon = (self.epsilon * self.epsilon_split).

     2) 1+ sketch is created from the union of all other frequencies. Here
     we noise 1+ sketch with epsilon = self.epsilon

  These two scenarios are controlled with the epsilon_split parameter. If
  epsilon_split = 0, then do scenario 1 otherwise do scenario 2.

[AaronRanAn]

Flagging that there's still an unresolved question from Craig.