Should "n/a" be allowed in the onset column of events.tsv

[VisLab]

The current description of onset in Task events says that the onset column may have 0, positive and negative values. It does not explicitly say it does not allow "n/a", although I believe that somewhere in the past that was in the spec. The current description of duration explicitly says that it can be "n/a" and what that means.

My question is --- is "n/a" really allowed in the onset column. If so, what does it mean and how are tools required to handle it?

This should be made explicit in the specification and in the schema.

This issue is related to issue #1938 and to issue hed-standard/hed-python#1026.