function Stratify(params) {
    Transform.call(this, null, params);
  }
Stratify a collection of tuples into a tree structure based on
id and parent id fields.
@constructor
@param {object} params - The parameters for this operator.
@param {function(object): *} params.key - Unique key field for each tuple.
@param {function(object): *} params.parentKey - Field with key for parent tuple.