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.

[samreid]

We are discussing using an {Emitter} instead of a {string}, defaulting to the PHET_CORE Timer emitter, in phetsims/twixt#19. I'll put the issue on hold until that is complete.

[samreid]

Since this carousel is decoupled from the model, it can run using the default value of timer. Closing.