Branch: CompositeStuckBalls

[brandonLi8]

From #87,

I said:

This makes me think that it might be better to add the 2 balls to a data structure that represents the 2 stuck balls.

I'm trying this in this branch.

[brandonLi8]

In https://github.com/phetsims/collision-lab/issues/87#issuecomment-647045167, I've outlined some questions before merging this into master.

However, there are still some implementation details to cover as well.

• [x] I really don't like the CompositeStuckBalls name.

Some other names to consider:

InelasticRotationEngine
InelasticRotationsEngine
StuckBallsCluster
StickyBallsRotator
StickyBallsHandler
StickyBallPairHandler
StickyBallPair

Ended up going with both StickyBallCluster and InelasticRotationEngine.

• [x] When one of the balls of the cluster is user-controlled, fully remove the cluster.
• [x] Clean up usage of CompositeStuckBalls
• [x] Reconfigure changed defaults
• [x] rename ball.prototype.step to ball.prototype.stepLinear or something
• [x] Update documentation in various places.

[brandonLi8]

I think at this point, this branch can be merged to master. Doing that now.

[brandonLi8]

Done. Closing.