Use a tolerance threshold of 1e-3 when determining
if nodes were moved externally since the last timestep.

This avoids precision issues when subtracting float values.

(Also, set timestep back down to 1e-5.)
Provide some access to the internal simulator
(friction, starting, stopping) in the physical Pane classes.
Create a VBoxExample based on (but not inheriting from)
the standard example.

The abstract class cannot be reused because the VBox uses its
own internal simulator.