Update wavelengthIndex description for processed data

[sreekanthkura7]

Clarified requirements for wavelengthIndex in processed data.

Closes #174

[HanBnrd]

Thanks @sreekanthkura7, good catch! Looks good to me!

[Horschig]

I guess this change would require a respective adjustment in the validator, right? Is @sstucker still the designated validator maintainer?

[jcrdubois]

is this the final convention that has been decided on? Some readers have made their own convention, e.g. Cedalion uses -1 https://github.com/ibs-lab/cedalion/blob/main/src/cedalion/io/snirf.py#L1003

[HanBnrd]

I guess this change would require a respective adjustment in the validator, right? Is @sstucker still the designated validator maintainer?

The validator has a bit of backlog on the specs, we could sort this as part of #183

is this the final convention that has been decided on? Some readers have made their own convention, e.g. Cedalion uses -1 https://github.com/ibs-lab/cedalion/blob/main/src/cedalion/io/snirf.py#L1003

@sreekanthkura7 could we do -1 instead of 0 eventually?