OpenEphysBinaryRawIO: Fixing ttl multichan

[vigji]

Addressing #1437.

Currently, all the logic in the events parser for OpenEphys data assumes that there is a single stream of events (simply positive or negative states), and it breaks down when acquiring multiple digital signals at once (states having different absolute values for different channels).

This is a simple fix that looks for rising and falling edges separately for each of the digital channels.

Add simple testing relying on existing test data, which fails before the fix and passes afterwards.

[alejoe91]

Thanks @vigji! Looking great!

[vigji]

Great, let me know if there's anything needed before merging!

[zm711]

This looks good to me, although I would ask for one additional comment in the code to explain what version this works for (all or v0.6+ whatever) along with what the structure of the data is.

[zm711]

This works for me, but Alessio knows this way better so I'll let him decide when to merge/do final review.