function GeoPoint(params) {
    Transform.call(this, null, params);
  }
Geo-code a longitude/latitude point to an x/y coordinate.
@constructor
@param {object} params - The parameters for this operator.
@param {function(number, number): *} params.projection - The cartographic
  projection to apply.
@param {Array<function(object): *>} params.fields - A two-element array of
  field accessors for the longitude and latitude values.
@param {Array<string>} [params.as] - A two-element array of field names
  under which to store the result. Defaults to ['x','y'].