function Lookup(params) {
    Transform.call(this, {}, params);
  }
Extend tuples by joining them with values from a lookup table.
@constructor
@param {object} params - The parameters for this operator.
@param {Map} params.index - The lookup table map.
@param {Array<function(object): *} params.fields - The fields to lookup.
@param {Array<string>} params.as - Output field names for each lookup value.
@param {*} [params.default] - A default value to use if lookup fails.