[ENH] Support for concurrently-acquired data with scans.tsv

[m-miedema]

Improves guidelines for using the scans.tsv file to identify simultaneously-acquired data. This PR closes #2362, following helpful discussion with @neuromechanist.

Notable changes:

• Adds documentation for usage of scans.tsv file in cases where multiple modalities or recordings are simultaneously acquired.
• Facilitates the use of the duration column along with acq_time to identify overlapping recordings by changing the definition of duration to explicitly cover recording length as well as event length
• Changes requirement levels of scans.tsv, acq_time, and duration from OPTIONAL to REQUIRED when multiple recordings are acquired concurrently.

Comments:
In proposing to make scans.tsv conditionally REQUIRED, we (i.e. the BEP 045 working group) acknowledge the additional complexity that is introduced for validation. However, we see the requirement of scans.tsv files as minimally burdensome for BIDS users and highly rewarding to facilitate automated processing of multimodal data. We are happy to discuss alternative approaches should this be deemed infeasible.

An example implementing these changes can be found here.

Thank you in advance for your attention and feedback!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.07%. Comparing base (308395f) to head (cd92084).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2441   +/-   ##
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  Coverage   83.07%   83.07%           
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  Files          22       22           
  Lines        1696     1696           
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  Hits         1409     1409           
  Misses        287      287           

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