feat(CellBudgetFile): support aux vars in get_ts() method

[wpbonelli]

Add a variable parameter to the CellBudgetFile.get_ts() method defaulting to "q". Close #2270.

[codecov]

Codecov Report

❌ Patch coverage is 26.92308% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.4%. Comparing base (556c088) to head (37e3695).
⚠️ Report is 34 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/utils/binaryfile/__init__.py	26.9%	19 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2568      +/-   ##
===========================================
+ Coverage     55.5%    72.4%   +16.9%     
===========================================
  Files          644      667      +23     
  Lines       124135   128619    +4484     
===========================================
+ Hits         68947    93246   +24299     
+ Misses       55188    35373   -19815     

Files with missing lines	Coverage Δ
flopy/utils/binaryfile/__init__.py	82.0% <26.9%> (-2.5%)	⬇️

... and 411 files with indirect coverage changes

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[wpbonelli]

@cneyens is this close to what you had in mind?

[cneyens]

@cneyens is this close to what you had in mind?

Yes, this looks great!