fix(mf6): opt into writing optional default values

[wpbonelli]

Add an option write_defaults to the simulation, determining whether optional options block fields that have the default value are written to input files.

Default to false to keep new/breaking options from being written to input files when they are not explicitly used. This gives new versions of flopy, aimed at new versions of MF6, better odds of remaining compatible with older versions of MF6. This is safe assuming MF6 consistently implements defaults as declared in DFN files. It will change the contents of input files, but in a way that prevents breakage, so seems better to consider a fix than a breaking change.

One can opt into writing all option fields regardless of value by setting write_defaults = True. This preserves the input file as a complete record of the user's intent.

Fix #2710

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (c5ba01d).
⚠️ Report is 130 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2712      +/-   ##
===========================================
+ Coverage     55.5%    72.4%   +16.9%     
===========================================
  Files          644      674      +30     
  Lines       124135   130572    +6437     
===========================================
+ Hits         68947    94635   +25688     
+ Misses       55188    35937   -19251     

Files with missing lines	Coverage Δ
flopy/mf6/data/mfdatascalar.py	60.4% <100.0%> (+1.0%)	⬆️
flopy/mf6/data/mfstructure.py	74.2% <100.0%> (+0.4%)	⬆️
flopy/mf6/mfsimbase.py	62.7% <100.0%> (-12.8%)	⬇️

... and 567 files with indirect coverage changes

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