bind (ev, f) {
    const old = this.events[ev]
    this.events[ev] = f
    return old
  }
Attaches a callback function to an event.
@function module:svgcanvas.SvgCanvas#bind
@param  {string} ev - String indicating the name of the event
@param {module:svgcanvas.EventHandler} f - The callback function to bind to the event
@returns {module:svgcanvas.EventHandler} The previous event