feat(mf6): opt out of writing default values

[wpbonelli]

Add a simulation configuration setting write_defaults=True, toggling whether to include optional fields that have the default value when writing input files. Default to True so this is opt-in, non-breaking. Setting this False can make input files written by FloPy more likely to be compatible with older versions of MF6, at the price of uncertain behavior as defaults may change.

Address #2710

[codecov]

Codecov Report

❌ Patch coverage is 68.96552% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.3%. Comparing base (556c088) to head (e115db5).
⚠️ Report is 205 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/data/mfdatascalar.py	57.1%	9 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2712      +/-   ##
===========================================
+ Coverage     55.5%    73.3%   +17.8%     
===========================================
  Files          644      659      +15     
  Lines       124135   132238    +8103     
===========================================
+ Hits         68947    97040   +28093     
+ Misses       55188    35198   -19990     

Files with missing lines	Coverage Δ
flopy/mf6/data/mfstructure.py	74.2% <100.0%> (+0.5%)	⬆️
flopy/mf6/mfsimbase.py	62.9% <100.0%> (-12.6%)	⬇️
flopy/mf6/data/mfdatascalar.py	58.8% <57.1%> (-0.5%)	⬇️

... and 582 files with indirect coverage changes

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