fix(HeadUFile): disable get_alldata without layer

[wpbonelli]

Close #1502. The issue is that get_alldata() assumes regular structure that USG files don't have. We could return e.g. a list of arrays or some other ragged representation but I figure better to acknowledge the limitation clearly than to return something technically valid but confusing. So, block usage of the method without providing a layer.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.3%. Comparing base (556c088) to head (21c5698).
⚠️ Report is 121 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/binaryfile/__init__.py	75.0%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2686      +/-   ##
===========================================
+ Coverage     55.5%    72.3%   +16.8%     
===========================================
  Files          644      667      +23     
  Lines       124135   130292    +6157     
===========================================
+ Hits         68947    94280   +25333     
+ Misses       55188    36012   -19176     

Files with missing lines	Coverage Δ
flopy/utils/binaryfile/__init__.py	82.3% <75.0%> (-2.2%)	⬇️

... and 562 files with indirect coverage changes

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