fix(MfUsgWel): add get_empty() override to support WELLBOT parameter

[reneangermeyer]

• Fixes TypeError when loading MFUSG models with WELLBOT option
• Adds static method get_empty() that accepts wellbot parameter
• WELLBOT option was already parsed in pakbase.py but not handled in dtype creation
• Enables loading of USG-Transport models with well bottom elevations

Resolves issue where models using 'WELLBOT' option in WEL package would fail with: TypeError: ModflowWel.get_empty() got an unexpected keyword argument 'wellbot'

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.6%. Comparing base (556c088) to head (9ef5670).
⚠️ Report is 88 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mfusg/mfusgwel.py	91.6%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2656      +/-   ##
===========================================
+ Coverage     55.5%    72.6%   +17.1%     
===========================================
  Files          644      667      +23     
  Lines       124135   129356    +5221     
===========================================
+ Hits         68947    93994   +25047     
+ Misses       55188    35362   -19826     

Files with missing lines	Coverage Δ
flopy/mfusg/mfusgwel.py	93.3% <91.6%> (+1.4%)	⬆️

... and 559 files with indirect coverage changes

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[wpbonelli]

makes sense to have for wel since we have it on other usg packages, thanks @reneangermeyer