Add function to build BABY-1L geometry and corresponding test

[davidepettinari]

This PR introduces a new function, baby_geometry, to the toolbox for building the BABY-1L geometry.

Previously, the geometry definition was duplicated across individual simulation scripts, for example in BABY-1L-run-1.
By centralizing it in a single function, the codebase becomes simpler, cleaner, and more readable.

A dedicated test file, test_baby.py, has been added in the test/ directory to ensure that the BABY-1L geometry is correctly generated and behaves as expected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.22%. Comparing base (2569f9b) to head (e409e38).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
+ Coverage   87.11%   88.22%   +1.10%     
==========================================
  Files          19       20       +1     
  Lines        1506     1647     +141     
==========================================
+ Hits         1312     1453     +141     
  Misses        194      194              

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