ENH: Warn users if masks are not provided for registration and fitting

[jhlegarreta]

Warn users if masks are not provided for registration and model fitting.

Closes #10.

[jhlegarreta]

Will add testing whenever we have some testing data.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 65.91%. Comparing base (6b6ba70) to head (d1acb49).

Files with missing lines	Patch %	Lines
src/nifreeze/model/base.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
+ Coverage   65.84%   65.91%   +0.07%     
==========================================
  Files          18       18              
  Lines         931      936       +5     
  Branches      119      120       +1     
==========================================
+ Hits          613      617       +4     
  Misses        274      274              
- Partials       44       45       +1     

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