[ENH] Specify that dir-<label> should correspond to PhaseEncodingDirection

[effigies]

The BIDS convention is to use dir-{AP,PA,LR,RL} for phase-encoding directions in MRI filenames, as well as PhaseEncodingDirection in JSON sidecars. This duplication has the potential for inconsistency or multiple interpretations of the meaning of AP.

This PR aims to clarify the meaning of dir-<label> when its value matches a cardinal axis, and clearly state that this meaning SHOULD be consistent with the PhaseEncodingDirection.

[oesteban]

This is great to make the situation better without introducing backward incompatible changes. I made some suggestions to be more explicit about what the problem is and providing examples, that said, the phrasing will likely require improvements.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.06%. Comparing base (9aaf7e3) to head (a5bad79).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2097   +/-   ##
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  Coverage   82.06%   82.06%           
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  Files          17       17           
  Lines        1533     1533           
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  Hits         1258     1258           
  Misses        275      275           

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