function Formula(params) {
    Transform.call(this, null, params);
  }
Invokes a function for each data tuple and saves the results as a new field.
@constructor
@param {object} params - The parameters for this operator.
@param {function(object): *} params.expr - The formula function to invoke for each tuple.
@param {string} params.as - The field name under which to save the result.
@param {boolean} [params.initonly=false] - If true, the formula is applied to
  added tuples only, and does not update in response to modifications.