try verlet integration

[annxingyuan]

Hey @anvaka - thank you so much for this library - it's been unbelievably helpful and I use it every day :)

I experimented with using Verlet integration rather than Euler integration to update the particle positions. You can't see much of a difference in the demo for this repo, but I'm using this package for larger network layouts, and I've found Verlet integration dramatically reduces jitter with the particles.

To show you what I mean, here's a video of the graph I'm laying out with your forcelayout3d module (using Euler integration):
https://www.youtube.com/watch?v=84gb8BwmRLU&feature=youtu.be

And here it is again with Verlet integration swapped in:
https://www.youtube.com/watch?v=YgHrCLeSqdQ&feature=youtu.be

Let me know what you think. Thank you again for all your work on ngraph.

[anvaka]

First of all, thank you for doing this!

Let me take a closer look over the weekend and merge this PR!

[anvaka]

I merged your changes. I noticed that Verlet integration takes longer to converge but indeed is much smoother than Euler schema. Also for larger time steps Verlet integration was not stable.

I'd like to keep both schemas available. The module uses Euler method by default, but if you pass a configuration object, it will switch to Verlet:

var layout = require('ngraph.forcelayout3d')(graph, {
 integrator: 'verlet'
});

Hope this compromise works well for you and old clients of the library. The change is published as v0.0.16

Thanks again for contributing!