fix(zonebudget_example): index the budget values by position

[jdhughes-dev]

volumetric_budget_bar_plot labels each bar by indexing the values with the position of the bar, but the values are passed in as a series indexed by zone name, so the label lookup raised KeyError: 0. Pandas used to fall back to positional indexing for a series that is not indexed by integers and no longer does. The values are now taken out of the series before they are indexed.

This is one of the notebooks failing in the nightly example workflow. The notebook runs to completion with this change and raises the KeyError without it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.8%. Comparing base (556c088) to head (a59cfc7).
⚠️ Report is 191 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2801      +/-   ##
===========================================
+ Coverage     55.5%    72.8%   +17.2%     
===========================================
  Files          644      662      +18     
  Lines       124135   133024    +8889     
===========================================
+ Hits         68947    96851   +27904     
+ Misses       55188    36173   -19015     

see 583 files with indirect coverage changes

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