[WIP] BEP020 Eyetracking

[rwblair]

Based off google doc.

todo/issues:

• json schema does not have correct types for any of the object of objects, from the examples I could see some of these may be arrays of arrays of strings.
• file regex does not include .asc as this was not in the google doc template but should be a valid template
• regex only allows for file in eyetrack/ this may change to allow eyetracking data appear next to any modality.
• template and example datasets have session in filename but not as a directory, validator throws and error for this as historically both were required and had to match for a given file name.
• events tsv validation logic will need to be updated if the above is allowed.
• events.tsv does not cross reference sidecar and _events.tsv eventIdentifier field to ensure they share the same values.
• json tests
• tsv tests
• filename tests

Will be able to resolve these as main specification PR progresses and bids-examples are added.

@mszinte Looking forward to the specification and examples PR. If you have any questions about whats going on here let me know.