function Flatten(params) {
    Transform.call(this, [], params);
  }
Flattens array-typed field values into new data objects.
If multiple fields are specified, they are treated as parallel arrays,
with output values included for each matching index (or null if missing).
@constructor
@param {object} params - The parameters for this operator.
@param {Array<function(object): *>} params.fields - An array of field
  accessors for the tuple fields that should be flattened.
@param {string} [params.index] - Optional output field name for index
  value. If unspecified, no index field is included in the output.
@param {Array<string>} [params.as] - Output field names for flattened
  array fields. Any unspecified fields will use the field name provided
  by the fields accessors.