fix(vtk): fix add_pathline_points prt data handling

[wpbonelli]

The logic to separate PRT pathlines by composite key for VTK export was

• slow: O(n^4 * m) where n is unique values per column and m is the total number of rows
• wrong: exported empty pathlines in most cases due to some incorrect filtering

Replace the nested loops and repeated np.unique() calls with a single unique(return_inverse=True) taking a composite key dtype. Should be O(n log n) now.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (8e87839).
⚠️ Report is 142 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2732      +/-   ##
===========================================
+ Coverage     55.5%    72.4%   +16.9%     
===========================================
  Files          644      674      +30     
  Lines       124135   130618    +6483     
===========================================
+ Hits         68947    94649   +25702     
+ Misses       55188    35969   -19219     

Files with missing lines	Coverage Δ
flopy/export/vtk.py	69.8% <100.0%> (-8.3%)	⬇️

... and 569 files with indirect coverage changes

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