evaluate stepper option to Carousel

[pixelzoom]

Carousel was converted to use TWIXT/Animation in phetsims/sun#381. The default is stepper: 'manual', which means that the client must call step to control the Carousel's animation. To avoid having to make changes in this sim, stepper: 'timer' was added to Carousel call sites, which means that the animation will run automatically using the sim Timer.

The responsible developer should evaluate what stepper value should be used.

[jbphet]

I've reviewed the change, and have decided to leave it as is, i.e. using the stepper: 'timer' option. This node is not currently being stepped in the implementation and I don't see much value in changing this, so I'll leave it as it was, where the animation behavior was autonomous. Closing.