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 looked at the code, and I think I'll leave the stepper value set to timer. This is pretty much how the original implementation worked, and the carousel isn't tracked as a property or variable inside AreaBuilderGameView, and it doesn't seem to me to be worthwhile to change this. I tested it on master and the carousel appears to work fine. Closing.