add parallel iterator for night identifier and trajectory identifier within an obsdataset

[FusRoman]

close #3

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 80.95238% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.57%. Comparing base (75e7477) to head (9e522fa).

Files with missing lines	Patch %	Lines
src/observation_dataset/parallel.rs	80.95%	8 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #4      +/-   ##
==========================================
- Coverage   92.63%   92.57%   -0.07%     
==========================================
  Files          35       35              
  Lines        8042     8084      +42     
==========================================
+ Hits         7450     7484      +34     
- Misses        592      600       +8     

Flag	Coverage Δ
rust	92.57% <80.95%> (-0.07%)	⬇️

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