onMouseWheel(event) {
    if (this.options.zoomView === true) {
      // If delta is nonzero, handle it.
      // Basically, delta is now positive if wheel was scrolled up,
      // and negative, if wheel was scrolled down.
      if (event.deltaY !== 0) {
        // calculate the new scale
        let scale = this.body.view.scale;
        scale *=
          1 + (event.deltaY < 0 ? 1 : -1) * (this.options.zoomSpeed * 0.1);

        // calculate the pointer location
        const pointer = this.getPointer({ x: event.clientX, y: event.clientY });

        // apply the new scale
        this.zoom(scale, pointer);
      }

      // Prevent default actions caused by mouse wheel.
      event.preventDefault();
    }
  }
Event handler for mouse wheel event, used to zoom the timeline
See http://adomas.org/javascript-mouse-wheel/
https://github.com/EightMedia/hammer.js/issues/256
@param {MouseEvent}  event
@private