fix(UcnFile): Add check for modflow-6 GWT concentration file header type

[jlarsen-usgs]

Closes #2559

[codecov]

Codecov Report

❌ Patch coverage is 87.87879% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (77b8f1e).
⚠️ Report is 132 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/binaryfile/__init__.py	87.8%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2718      +/-   ##
===========================================
+ Coverage     55.5%    72.4%   +16.9%     
===========================================
  Files          644      674      +30     
  Lines       124135   130612    +6477     
===========================================
+ Hits         68947    94651   +25704     
+ Misses       55188    35961   -19227     

Files with missing lines	Coverage Δ
flopy/utils/binaryfile/__init__.py	82.4% <87.8%> (-2.1%)	⬇️

... and 569 files with indirect coverage changes

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