start(world) {
    if (this.enabled) return;

    if (world) world.addLoop(this);

    if (this.clock) this.clock.start();
    this.enabled = true;
  }
@method start
@instance
@description Starts this loop, clock if it has one. Won't do anything if loop enabled already.
@param {Component} [world] app to add this loop to, if provided.
@memberof module:core.Loop