improve animations

[wonder-mice]

I'm creating this issue to brain storm ideas on how animations can be improved to run smoothly even for large data sets.

Problem is that d3 animates on CPU by updating DOM values "manually" via timer. While flexible, with large node count transition FPS drops dramatically. For my data set size I have 2-3 frames per 750ms transition.

Generally, this can tackled from following directions:

1. Animate nodes as a group, to avoid setting individual properties of each node
2. Distribute node transitions over time by sequentially animating subsets of nodes
3. Use simpler animations, like opacity change that doesn't require layout and rendering of individual nodes

[spiermar]

Not sure if 3 applies, since nodes will have to change size for zoom. 1 seems interesting, but need to investigate how to get it implemented using the current structure.