Revamped spike messages.

The SpikeDetector now uses SpikeObjects to pass spike messages,
rather than the GenericProcessor "addEvent" code, as previously.
This makes it easier to distribute spikes to multi-channel electrode
displays, such as StereotrodePlot and TetrodePlot. However, there's
still a problem with setting the axes. "Real" spikes have an issue
with constantly changing the axes, whereas simulated spikes do not.