function GeoPath(params) {
    Transform.call(this, null, params);
  }
Map GeoJSON data to an SVG path string.
@constructor
@param {object} params - The parameters for this operator.
@param {function(number, number): *} params.projection - The cartographic
  projection to apply.
@param {function(object): *} [params.field] - The field with GeoJSON data,
  or null if the tuple itself is a GeoJSON feature.
@param {string} [params.as='path'] - The output field in which to store
  the generated path data (default 'path').