fix(model_splitter): multiple bug fixes

[jlarsen-usgs]

• filter (-1,) SFR cellids from splitting mask creation in optimize_splitting_mask
• adjust (-1,) SFR cellid layer number to 0 in _remap_sfr to avoid index errors
• handle external model files by allowing user to specify new simulation workspace sim_ws to split_model and split_multi_model
• dynamically adjust max_columns_of_data when external model files are maintained in the split models

[jlarsen-usgs]

@jptraylor

Closes #2726

[codecov]

Codecov Report

❌ Patch coverage is 47.50000% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (29a6f49).
⚠️ Report is 138 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/utils/model_splitter.py	47.5%	21 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2728      +/-   ##
===========================================
+ Coverage     55.5%    72.4%   +16.9%     
===========================================
  Files          644      674      +30     
  Lines       124135   130674    +6539     
===========================================
+ Hits         68947    94673   +25726     
+ Misses       55188    36001   -19187     

Files with missing lines	Coverage Δ
flopy/mf6/utils/model_splitter.py	65.5% <47.5%> (-9.5%)	⬇️

... and 569 files with indirect coverage changes

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