STYLE: Use the same alias consistently to import nibabel

[jhlegarreta]

Use the same alias consistently to import nibabel: prefer using nib over nb as it is more descriptive.

[codecov]

Codecov Report

Attention: Patch coverage is 73.68421% with 5 lines in your changes missing coverage. Please review.

Project coverage is 65.84%. Comparing base (6b6ba70) to head (5b84043).

Files with missing lines	Patch %	Lines
src/nifreeze/data/pet.py	20.00%	4 Missing ⚠️
src/nifreeze/registration/utils.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #43   +/-   ##
=======================================
  Coverage   65.84%   65.84%           
=======================================
  Files          18       18           
  Lines         931      931           
  Branches      119      119           
=======================================
  Hits          613      613           
  Misses        274      274           
  Partials       44       44           

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