Stims based on BIDS events.tsv

[sstucker]

To return SNIRF to the comfortable "read-only" relationship with Homer it had until recently, I propose to rebuild the stims system on top of the BIDS-specified events.tsv record of stims.

This file can be generated from SNIRF files as necessary by data loading processes. The StimEditGUI of the future, incorporating #100 and #99, will edit only the events.tsv file, which will be conveted back to StimClass structure for processing.