Fixed TTL events causing Spike Viewer glitch

SpikeDetector used the channel count as eventType on the event channel list, which was used by SpikeDisplayNode to draw the appropiate plot. However, eventType should be used to differentiate different event types, as TTL vs. SPIKE. As result, SpikeDisplayNode tried to plot TTL event channels as 3 electrode plots.
Fixed by adding an offset (SPIKE_BASE_CODE=100) to the channel count at eventType to avoid said collision. However, we perhaps should try to find a more elegant solution.