Fix#7 - forces are now mutable, so it’s easier to change their settings and
they behave more like other configurable objects in D3 (though less like curves,
easing functions and interpolators).
Fix#9 - forces now have a dedicated interface rather than just being beforetick
listeners. This allows forces to be re-initialized when the nodes change.
Fix#10 - d3.forcePosition now initializes node positions and velocities.
