Update(model_splitter): add timeseries support

[jlarsen-usgs]

• add support for empty fmi package
• add support for buy package mapping

[codecov]

Codecov Report

Attention: Patch coverage is 35.48387% with 20 lines in your changes missing coverage. Please review.

Project coverage is 76.3%. Comparing base (bb9824e) to head (1bb508f).
Report is 27 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/utils/model_splitter.py	35.4%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2403     +/-   ##
=========================================
+ Coverage     68.4%   76.3%   +7.9%     
=========================================
  Files          294     294             
  Lines        59390   59733    +343     
=========================================
+ Hits         40652   45635   +4983     
+ Misses       18738   14098   -4640     

Files with missing lines	Coverage Δ
flopy/mf6/utils/model_splitter.py	71.9% <35.4%> (+13.1%)	⬆️

... and 245 files with indirect coverage changes