Moved fields from Simulation to PhysLayout.

Also fixed a few comments in the Fields.
Fields: Accept angle instead of vector
Clean up unused nt variable.
Implement start/stop for animation.

* The animation timer is now created immediately and stored.
* start() and stop() calls are passed through to it.
* The time step is also an object property and changed
  separately instead of as an argument to startSimulation().
Define some constants and a helper constructor.
public updateView() function.
Make "running" a javafx property.

It will be useful to be able to react to the status
of the simulation in other parts of the program.
Make example more interactive.

Add a toolbar with start/stop/step/randomize buttons.