Clean up API.

[donmccurdy]

Lots of little details that should be polished, like:

const node = pathfinding.getClosestNode(a, ZONE);
const groupID = pathfinding.getGroup(ZONE, a);

[donmccurdy]

On second thought, I'd like to explore https://github.com/donmccurdy/three-pathfinding/issues/34 before making any breaking changes to the API here.