function Quantile$1(params) {
    Transform.call(this, null, params);
  }
Generates sample quantile values from an input data stream.
@constructor
@param {object} params - The parameters for this operator.
@param {function(object): *} params.field - An accessor for the data field
  over which to calculate quantile values.
@param {Array<function(object): *>} [params.groupby] - An array of accessors
  to groupby.
@param {Array<number>} [params.probs] - An array of probabilities in
  the range (0, 1) for which to compute quantile values. If not specified,
  the *step* parameter will be used.
@param {Array<number>} [params.step=0.01] - A probability step size for
  sampling quantile values. All values from one-half the step size up to
  1 (exclusive) will be sampled. This parameter is only used if the
  *quantiles* parameter is not provided.