BUG: Fix the non-weighted DWI data serialization

[jhlegarreta]

WIP: Re issue
#79

Fixes #79.

[jhlegarreta]

This only demonstrates the issue; I wrote the code as I spent time investigating the issue, so pushing it here to move forward. Some thinking/discussion is needed to see what convention we follow so that the issue can be addressed properly.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 46.28%. Comparing base (0866a82) to head (9be0147).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nifreeze/data/dmri.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #82       +/-   ##
===========================================
- Coverage   70.58%   46.28%   -24.30%     
===========================================
  Files          21       21               
  Lines        1020     1024        +4     
  Branches      119      119               
===========================================
- Hits          720      474      -246     
- Misses        258      522      +264     
+ Partials       42       28       -14     

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