Add test for relative path in flats and fix reduced path basename

[Allonck]

• Added test for relative path in get_best_flat helper function
• Added fixes Dome flats are not being processed  #391

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.97%. Comparing base (1ede463) to head (7d55b40).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #392   +/-   ##
=======================================
  Coverage   79.97%   79.97%           
=======================================
  Files          15       15           
  Lines        2642     2642           
=======================================
  Hits         2113     2113           
  Misses        529      529           

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