fix(mfdatascalar): skip multiple choice fields with default values

[wpbonelli]

When writing MF6 input files, don't write optional scalar variables whose values are equal to the defaults as specified in DFNs. This is a backwards-compatibility fix meant to keep new/breaking options from being written to input files when they are not explicitly used, so that new versions of flopy remain compatible with older versions of MF6.

This could cause significant differences in input files, but should not cause behavioral differences as MF6 implements the default values internally.

Introduce an option write_defaults on the simulation, to allow opting into the old behavior. I figure it can still be useful as a kind of 'strict' mode.

Fix #2710

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (bb9e1cc).
⚠️ 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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