update(ModflowWel): add basic support for mfnwt tabfile based well files

[jlarsen-usgs]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.5%. Comparing base (556c088) to head (1367c9d).
⚠️ Report is 159 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##           develop    #2755     +/-   ##
==========================================
- Coverage     55.5%    52.5%   -3.0%     
==========================================
  Files          644      677     +33     
  Lines       124135   132656   +8521     
==========================================
+ Hits         68947    69726    +779     
- Misses       55188    62930   +7742     

Files with missing lines	Coverage Δ
flopy/modflow/mfwel.py	88.1% <100.0%> (-1.9%)	⬇️
flopy/pakbase.py	82.8% <100.0%> (-1.7%)	⬇️

... and 518 files with indirect coverage changes

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