update(_remap_sfr): add more robust handling for (0, 0, 0) SFR cells

[jlarsen-usgs]

• add logic to traverse the connectiondata graph to find the nearest
  connected GWF/SFR cell in which (0, 0, 0) cells are routed to

[jlarsen-usgs]

Will add testing soon to wrap up the development on this PR

[codecov]

Codecov Report

❌ Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.6%. Comparing base (556c088) to head (789e0b6).
⚠️ Report is 226 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/utils/model_splitter.py	0.0%	15 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2840      +/-   ##
===========================================
+ Coverage     55.5%    73.6%   +18.0%     
===========================================
  Files          644      659      +15     
  Lines       124135   132300    +8165     
===========================================
+ Hits         68947    97394   +28447     
+ Misses       55188    34906   -20282     

Files with missing lines	Coverage Δ
flopy/mf6/utils/model_splitter.py	68.6% <0.0%> (-6.3%)	⬇️

... and 585 files with indirect coverage changes

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