Implement MultiShellKernel

[oesteban]

Summary

• introduce MultiShellKernel to model multi-shell diffusion data
• tidy imports and wrap long docstrings

Testing

• ruff check src/nifreeze/model/gpr.py
• pytest test/test_gpr.py -q

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https://chatgpt.com/codex/tasks/task_e_6880ea2776988330816c1d690bc19670

[codecov]

Codecov Report

❌ Patch coverage is 28.12500% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.96%. Comparing base (55b1c68) to head (94a6838).
⚠️ Report is 75 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nifreeze/model/gpr.py	28.12%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
- Coverage   73.98%   72.96%   -1.03%     
==========================================
  Files          24       24              
  Lines        1403     1435      +32     
  Branches      166      168       +2     
==========================================
+ Hits         1038     1047       +9     
- Misses        299      322      +23     
  Partials       66       66              

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