update(model_splitter.py): support for SSM and ATS

[jlarsen-usgs]

• fix support for SSM file mapping
• add ATS support
• replace match/case statements in ModelTime with if/else

Closes #2497

[codecov]

Codecov Report

Attention: Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.

Project coverage is 56.1%. Comparing base (7eb7c88) to head (7c0f1eb).
Report is 14 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/utils/model_splitter.py	92.3%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2505      +/-   ##
===========================================
- Coverage     76.1%    56.1%   -20.1%     
===========================================
  Files          296      640     +344     
  Lines        64215   124357   +60142     
===========================================
+ Hits         48885    69775   +20890     
- Misses       15330    54582   +39252     

Files with missing lines	Coverage Δ
flopy/discretization/modeltime.py	76.9% <100.0%> (-0.2%)	⬇️
flopy/mf6/utils/model_splitter.py	75.0% <92.3%> (+<0.1%)	⬆️

... and 474 files with indirect coverage changes

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