update(_remap_array): support non-layered external array data

[jlarsen-usgs]

• Add entry point to "layered" data specification in MFArray

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (0182d97).
⚠️ Report is 154 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/data/mfdataarray.py	60.0%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2747      +/-   ##
===========================================
+ Coverage     55.5%    72.4%   +16.8%     
===========================================
  Files          644      674      +30     
  Lines       124135   132024    +7889     
===========================================
+ Hits         68947    95608   +26661     
+ Misses       55188    36416   -18772     

Files with missing lines	Coverage Δ
flopy/mf6/utils/model_splitter.py	65.5% <100.0%> (-9.4%)	⬇️
flopy/mf6/data/mfdataarray.py	61.8% <60.0%> (-5.3%)	⬇️

... and 570 files with indirect coverage changes

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