[ENH] apply derivative definition to qMRI

[Remi-Gau]

• closes #1812

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.92%. Comparing base (0950f6d) to head (a653a18).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1813 +/- ## ======================================= Coverage 87.92% 87.92% ======================================= Files 16 16 Lines 1375 1375 ======================================= Hits 1209 1209 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.