[ENH] Apply derivative definition to qMRI appendix

[Remi-Gau]

• closes update qMRI to align with the new definition of derivatives #1812

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• update qMRI sections that mention qmaps: https://bids-specification--1813.org.readthedocs.build/en/1813/appendices/qmri.html#outputs-are-quantitative-maps

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.10%. Comparing base (7b0f76d) to head (39c6cfc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1813   +/-   ##
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  Coverage   88.10%   88.10%           
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  Files          16       16           
  Lines        1396     1396           
=======================================
  Hits         1230     1230           
  Misses        166      166           

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[Remi-Gau]

Will create a companion PR to modify a couple of bids example to make that the validator has got something to sink its teeth into.

[Remi-Gau]

@agahkarakuzu can you maybe cross-check what I did here?

[agahkarakuzu]

@Remi-Gau the changes you introduced looks good to me! This was initially what we wanted to do, but at that time it was not really clear. Thank you!