[FIX] Add definitions for SampleCoordinateSystem values

[sinaklg]

[FIX] Add definitions for SampleCoordinateSystem enum values

Adds definitions for the four SampleCoordinateSystem enum values (gaze-on-screen, eye-in-head, gaze-in-world, custom), which were previously listed without explanation, following the same inline-definition pattern already used for EyeTrackingMethod

Closes #2474

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.09%. Comparing base (4523097) to head (40437d6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2477   +/-   ##
=======================================
  Coverage   83.09%   83.09%           
=======================================
  Files          22       22           
  Lines        1698     1698           
=======================================
  Hits         1411     1411           
  Misses        287      287           

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