Replace os with pathlib

[henry2004y]

• Replace os with pathlib internally for file handling

[codecov]

Codecov Report

Attention: Patch coverage is 90.56604% with 5 lines in your changes missing coverage. Please review.

Project coverage is 55.62%. Comparing base (a898644) to head (4606075).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/flekspy/tp/test_particles.py	76.19%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   55.57%   55.62%   +0.05%     
==========================================
  Files          12       12              
  Lines        1686     1688       +2     
==========================================
+ Hits          937      939       +2     
  Misses        749      749              

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