EventDetection does not support storing the channel of the spike event

[rly]

See NeurodataWithoutBorders/pynwb#2068

EventDetection stores indices into the first dimension of the linked ElectricalSeries data array. But ElectricalSeries can have multiple channels along the second dimension, and it is useful to store which channel a spike event was detected on.

Secondly, the times dataset of EventDetection can easily be computed from the linked ElectricalSeries (based on either indexing into the timestamps array or computing with starting time and sampling rate) and should therefore be optional.