function Impute(params) {
    Transform.call(this, [], params);
  }
Impute missing values.
@constructor
@param {object} params - The parameters for this operator.
@param {function(object): *} params.field - The value field to impute.
@param {Array<function(object): *>} [params.groupby] - An array of
  accessors to determine series within which to perform imputation.
@param {function(object): *} params.key - An accessor for a key value.
  Each key value should be unique within a group. New tuples will be
  imputed for any key values that are not found within a group.
@param {Array<*>} [params.keyvals] - Optional array of required key
  values. New tuples will be imputed for any key values that are not
  found within a group. In addition, these values will be automatically
  augmented with the key values observed in the input data.
@param {string} [method='value'] - The imputation method to use. One of
  'value', 'mean', 'median', 'max', 'min'.
@param {*} [value=0] - The constant value to use for imputation
  when using method 'value'.