function Loess(params) {
    Transform.call(this, null, params);
  }
Compute locally-weighted regression fits for one or more data groups.
@constructor
@param {object} params - The parameters for this operator.
@param {function(object): *} params.x - An accessor for the predictor data field.
@param {function(object): *} params.y - An accessor for the predicted data field.
@param {Array<function(object): *>} [params.groupby] - An array of accessors to groupby.
@param {number} [params.bandwidth=0.3] - The loess bandwidth.