removeNode(): functional op.
Force field example:

This example has four point-shaped attractors
that pull on the red ball.

When the ball is moved from its unstable equilibrium
position, it will be attracted toward the nearest blue dot.
Merge branch 'master' into multisprings

Conflicts:
	src/main/java/layout/PhysLayout.java
Add an example for multiple springs.

There are two balls connected by many springs with
random anchors. Individually, the springs only seek
to maintain equilibrium length, and rotate freely.

Taken together, the springs act like a flexible rod
that forces the two balls to maintain both relative
distance and angle.

Moving the balls and reconnecting will make their current
relative position the new equilibrium.