fix: auto-set max_columns_of_data for external writes

[wpbonelli]

Fix the issue described in #2572 (comment) so max_columns_of_data is auto-set to ncol for all DIS models regardless whether arrays are written internally or externally. This change does not address DISV/DISU.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.5%. Comparing base (556c088) to head (c1980c7).
⚠️ Report is 173 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2771      +/-   ##
===========================================
+ Coverage     55.5%    72.5%   +17.0%     
===========================================
  Files          644      677      +33     
  Lines       124135   132710    +8575     
===========================================
+ Hits         68947    96340   +27393     
+ Misses       55188    36370   -18818     

Files with missing lines	Coverage Δ
flopy/mf6/mfmodel.py	57.3% <100.0%> (-23.6%)	⬇️
flopy/mf6/mfsimbase.py	62.8% <100.0%> (-12.6%)	⬇️

... and 578 files with indirect coverage changes

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