update(Mf6Splitter): update data types for memory reduction

[jlarsen-usgs]

• update ._node_map and ._connection representation from dictionary to numpy arrays to reduce memory usage
• add ._node_map_arr and ._model_map_arr to store remapping information
• store ._connection information as iac, ja representation
• add no_remap_key to force model splitter not to remap inactive cells from original model when active_only is used to create splitting mask

[jlarsen-usgs]

Pull request will reduce memory consumption and help preserve compact splitting from metis as noted #2813 and #2814

[codecov]

Codecov Report

❌ Patch coverage is 81.33333% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.4%. Comparing base (556c088) to head (4f929ea).
⚠️ Report is 209 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/utils/model_splitter.py	81.3%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2819      +/-   ##
===========================================
+ Coverage     55.5%    73.4%   +17.9%     
===========================================
  Files          644      659      +15     
  Lines       124135   132174    +8039     
===========================================
+ Hits         68947    97090   +28143     
+ Misses       55188    35084   -20104     

Files with missing lines	Coverage Δ
flopy/mf6/utils/model_splitter.py	65.8% <81.3%> (-9.1%)	⬇️

... and 584 files with indirect coverage changes

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[jlarsen-usgs]

Additional work on #2816 will come as a separate PR