Consider removing support for multiple /nirs{i} blocks: for many applications, a SNIRF file should only contain one acquisition from a single subject

[sstucker]

We (@dboas, @rob-luke) are making an effort to make SNIRF agree with the BIDS dataset.

The BIDS specification is dependent on acquisition files belonging to a single subject, session and "run"-- with multiple runs being broken up into several files.

This means that SNIRF files that include multiple /nirs{i} groups are not BIDS compliant.

It was suggested that @fangq saves files with multiple /nirs{i} groups?

We don't want to break backwards compatibility, but wanted to discuss here the motivation for supporting /nirs{i} blocks and encouraging their use in the future.