WIP ENH: Add event histogram example

I have some failing tests, not yet ready for review.
`add_events` can, sure, but this is actually true of any type of event during an experiment. `STI 014` (or now, `STI101`) is a sum of the binary-to-decimal conversion of the other channels, so if one has two truly concurrent events (e.g., a stimulus onset coded by "1" and a button press coded by "2") and they happen simultaneously, a single value ("3") will be coded at that time.