Fix #20 - simulation.tick now returns boolean.
Fix #20 - simulation.tick now returns boolean.

This changes simulation.tick to always advance the simulation one step,
regardless of the current value of alpha and alphaMin. If alpha is less than
alphaMin after this step, simulation.tick returns true, indicating that the
simulation can be safely ended.

Furthermore, simulation.tick no longer dispatches a tick event; the tick event
is only dispatched by the simulation’s internal timer. This is now consistent
with simulation.stop, which likewise does not dispatch an end event; the end
event is only dispatched when the simulation’s internal timer stops.

d3/d3-force