function getRadialLayoutMargin(width, height, radius) {
  const marginX = width / 2 - radius;
  const marginY = height / 2 - radius;
  return {
    bottom: marginY,
    left: marginX,
    right: marginX,
    top: marginY
  };
}
Calculate the margin of the sunburst,
so it can be at the center of the container
@param  {Number} width - the width of the container
@param  {Number} height - the height of the container
@param  {Number} radius - the max radius of the sunburst
@return {Object} an object includes {bottom, left, right, top}