function ingest(target, data, format) {
    data = this.parse(data, format);
    return this.pulse(target, this.changeset().insert(data));
  }
Ingests new data into the dataflow. First parses the data using the
vega-loader read method, then pulses a changeset to the target operator.
@param {Operator} target - The Operator to target with ingested data,
  typically a Collect transform instance.
@param {*} data - The input data, prior to parsing. For JSON this may
  be a string or an object. For CSV, TSV, etc should be a string.
@param {object} format - The data format description for parsing
  loaded data. This object is passed to the vega-loader read method.
@returns {Dataflow}