fix(mfdataarray): use integer indexed filenames

[wpbonelli]

As suggested in #2702 (comment). Numbering is 1-based as elsewhere

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (d3e8fc1).
⚠️ Report is 125 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2704      +/-   ##
===========================================
+ Coverage     55.5%    72.4%   +16.9%     
===========================================
  Files          644      669      +25     
  Lines       124135   130532    +6397     
===========================================
+ Hits         68947    94582   +25635     
+ Misses       55188    35950   -19238     

Files with missing lines	Coverage Δ
flopy/mf6/data/mfdataarray.py	61.8% <100.0%> (-5.2%)	⬇️

... and 564 files with indirect coverage changes

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