ENH: Add a shell data property to DWI data class

[jhlegarreta]

Add a shell data property to DWI data class that returns a list of pairs consisting of the estimated b-value and the associated DWI data.

Closes #66.

[jhlegarreta]

Opened as draft as I cannot debug this locally due to issue #73.

Edit: There seems to be some data formatting to be done to compare the DWI data obtained to the expected data, but the approach seems to be working otherwise:
https://github.com/nipreps/nifreeze/actions/runs/12969669089/job/36173994765?pr=74#step:12:1064
https://github.com/nipreps/nifreeze/pull/74/files#diff-f311d147ca2f3984f30068c37db4ca0a780e387581219cd6fa50a33e917d12cbR204

Edit: managed to debug locally undoing the type hinting related stuff. Ready to be reviewed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.91%. Comparing base (9a143e4) to head (ad46aa3).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #74      +/-   ##
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+ Coverage   68.78%   68.91%   +0.12%     
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  Files          20       20              
  Lines         958      962       +4     
  Branches      121      121              
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+ Hits          659      663       +4     
  Misses        254      254              
  Partials       45       45              

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[jhlegarreta]

Maybe it is worthwhile thinking about the interplay between this feature and the b-value iterators, and how the whole thing fits into the estimators' pipelining philosophy.

[oesteban]

Maybe it is worthwhile thinking about the interplay between this feature and the b-value iterators, and how the whole thing fits into the estimators' pipelining philosophy.

I agree. In our last iteration we made the iterators independent of the object, so now you could go shell by shell and run any available iterator (those without concept of b-value, evidently).

[oesteban]

Can we use the bvecs/bvals files we added for testing? They are within the repo (i.e., no need to pull extra data from GIN) and they provide several use-cases ideal for testing this.