Allow arbor to deal with a single node.

[max333]

Made changes so that arbor can deal with a single node. There are 3 different changes, all in physics.js:

1. Barnes-Hut was getting in an infinite loop with a single node. It was not even possible to add more nodes. Now Barnes-Hut does nothing when there are less than two nodes.
2. A single node used to jitter, but now applyCenterDrift does nothing for less than two nodes.
3. The bounding box would get reduced to a point for a single node, but now the bounding box is unchanged for less two nodes.