smoother mothion for node group collapse and expansion

[tedchwang]

Is your feature request related to a problem? Please describe.
Currently the dependency graph of a model produces violent motions when a node group is collapsed or expanded, messing up the node layout that I had on screen, and making it difficult to follow which part of the graph has changed.

Describe the solution you'd like
Set forces to be very small and gradually increase them to the standard strength.

Describe alternatives you've considered
Setting link distences to be small and gradually increasing them. This is the current behavior and it does not produce good enough results.

[tedchwang]

Perhaps:

• add a "newlyEntered" class to nodes in newly expanded node groups
• make the "collide" force and link force strengths depend on d.newlyEntered and simulation.alpha
• remove "newlyEntered" class after simulation stops
• see if we can skip labeling links that have one end changed from node to group (or group to node) not be considered as newlyEntered