feat: support nwt to mf6 binary output file conversion

[wpbonelli]

Support converting MODFLOW-NWT binary outputs to MF6 binary output files, for compatibility with use cases requiring MF6 outputs (e.g. MF6 PRT). Introduce a class NwtToMf6Converter to handle the conversion.

Required a few utilities:

• get_icelltype_from_laytyp(): convert NWT LAYTYP to MF6 ICELLTYPE
• get_saturation(): calculate saturation from head, cell geometry, and cell type
• get_structured_flowja(): inverse of get_structured_faceflows()
• MfGrdFile.write_dis(): write grb file for structured grid

[codecov]

Codecov Report

❌ Patch coverage is 79.75309% with 82 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.5%. Comparing base (556c088) to head (8c56314).
⚠️ Report is 151 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/nwt_to_mf6.py	77.7%	33 Missing ⚠️
flopy/mf6/utils/binarygrid_util.py	74.7%	27 Missing ⚠️
flopy/utils/postprocessing.py	80.9%	12 Missing ⚠️
flopy/mf6/utils/postprocessing.py	88.2%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2746      +/-   ##
===========================================
+ Coverage     55.5%    72.5%   +16.9%     
===========================================
  Files          644      676      +32     
  Lines       124135   132800    +8665     
===========================================
+ Hits         68947    96299   +27352     
+ Misses       55188    36501   -18687     

Files with missing lines	Coverage Δ
flopy/mf6/utils/__init__.py	100.0% <100.0%> (ø)
flopy/utils/__init__.py	100.0% <100.0%> (ø)
flopy/mf6/utils/postprocessing.py	92.1% <88.2%> (-4.7%)	⬇️
flopy/utils/postprocessing.py	80.2% <80.9%> (+2.6%)	⬆️
flopy/mf6/utils/binarygrid_util.py	86.5% <74.7%> (-4.3%)	⬇️
flopy/utils/nwt_to_mf6.py	77.7% <77.7%> (ø)

... and 571 files with indirect coverage changes

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