fix: use absolute path resolution for file handling in ND2Reader

[tlambert03]

closes #258

[codspeed-hq]

CodSpeed Performance Report

Merging #269 will not alter performance

_{Comparing fix-258 (ccd3e53) with main (4dc6655)}

Summary

✅ 13 untouched

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.53%. Comparing base (4dc6655) to head (ccd3e53).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #269   +/-   ##
=======================================
  Coverage   94.53%   94.53%           
=======================================
  Files          20       20           
  Lines        2451     2451           
=======================================
  Hits         2317     2317           
  Misses        134      134           

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