Add animation for hopf rule

[RazinShaikh]

We can apply hopf rule, aka leg chopping, using the magic wand but this rewrite is missing an animation.

[jvdwetering]

What should the animation look like? The wires gradually getting thinner?

[RazinShaikh]

Doing something like the following would be very cool.

[jvdwetering]

That would make me feel like a ninja, so I approve. That seems complicated though. Maybe something: calculate intersection point with the slice, then replace each of the edges with two edges where the boundary point is at the slice point. Then move this point closer to the spider until you can remove it. However, the hard part would be figuring out how to curve the wires properly as they move back.