start(time) {
    if (!this.isPlaying) {
      this.isPlaying = true;
      this.metro.resetSync(this);
      var t = time || 0;
      this.metro.start(t);
    }
  }
Start playback of this part. It will play
 through all of its phrases at a speed
 determined by setBPM.

 @method  start
 @for p5.Part
 @param  {Number} [time] seconds from now