Fixing a typo in gdp.hull transformation

[emma58]

Fixes # .

Summary/Motivation:

This fixes a sneaky typo in gdp.hull having to do with making disaggregated variables local in nested GDPs.

Changes proposed in this PR:

• Fixes a typo in hull that I think we didn't catch before because of some creepy Python scoping (sometimes disaggregatedVar was a thing--I'm not sure what.)
• Adds a test.

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
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[mrmundt]

I'm mad about the fact that we had 20 PRs open twice today.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.40 ⚠️

Comparison is base (4f2d5fc) 87.82% compared to head (848ece7) 87.43%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2915      +/-   ##
==========================================
- Coverage   87.82%   87.43%   -0.40%     
==========================================
  Files         770      770              
  Lines       89637    89637              
==========================================
- Hits        78726    78375     -351     
- Misses      10911    11262     +351     

Flag	Coverage Δ
linux	84.38% <0.00%> (-0.52%)	⬇️
osx	74.44% <0.00%> (-0.19%)	⬇️
other	84.56% <0.00%> (-0.52%)	⬇️
win	81.96% <0.00%> (-0.20%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyomo/gdp/plugins/hull.py	93.52% <0.00%> (ø)

... and 39 files with indirect coverage changes

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