fix(cellbudgetfile): fix get_ts support for aux vars

[wpbonelli]

close #2647, correcting initial buggy implementation of #2270

[codecov]

Codecov Report

❌ Patch coverage is 78.78788% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.4%. Comparing base (556c088) to head (b01f91e).
⚠️ Report is 84 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/binaryfile/__init__.py	78.7%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2648      +/-   ##
===========================================
+ Coverage     55.5%    71.4%   +15.9%     
===========================================
  Files          644      667      +23     
  Lines       124135   129154    +5019     
===========================================
+ Hits         68947    92294   +23347     
+ Misses       55188    36860   -18328     

Files with missing lines	Coverage Δ
flopy/utils/binaryfile/__init__.py	83.8% <78.7%> (-0.7%)	⬇️

... and 559 files with indirect coverage changes

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