start(timeFromNow) {
    var t = timeFromNow || 0;
    var now = p5sound.audiocontext.currentTime;
    if (!this.isPlaying) {
      this.clock.start(now + t);
      this.isPlaying = true;
    }
  }
Start the loop
@method  start
@for p5.SoundLoop
@param  {Number} [timeFromNow] schedule a starting time