contrib.piecewise: Adding transformation to outer representation GDP

[emma58]

Fixes # .

Summary/Motivation:

This adds a transformation from a model containing PiecewiseLinearFunctions to a GDP where the domains underlying each piece of the piecewise linear function are represented using an "outer" (Ax <= b) representation. This allows for a wrapper transformation to create the multiple choice formulation from Vielma et al. 2010.

NOTE: this includes the changes in #2708, so it will be easier to review once that is merged.

Changes proposed in this PR:

• Creates a piecewise-linear-to-gdp-transformation base class since the only method that is unique to the different transformations is how we transform PiecewiseLinearExpressions themselves. This currently relies on scipy for multivariate functions.
• Adds the 'contrib.piecewise.outer_repn_gdp' transformation
• Centralizes common models and tests so there there isn't duplicated code there either. (Let it not be said that Emma never learns from her mistakes!)

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[codecov]

Codecov Report

Patch coverage: 95.56% and no project coverage change.

Comparison is base (cf3fb9b) 87.00% compared to head (8164d7a) 87.01%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2766   +/-   ##
=======================================
  Coverage   87.00%   87.01%           
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  Files         758      761    +3     
  Lines       86991    87057   +66     
=======================================
+ Hits        75690    75751   +61     
- Misses      11301    11306    +5     

Flag	Coverage Δ
linux	84.01% <95.56%> (+0.01%)	⬆️
osx	73.49% <95.56%> (+0.01%)	⬆️
other	84.19% <95.56%> (+0.01%)	⬆️
win	81.52% <95.56%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...ewise/transform/piecewise_to_gdp_transformation.py	92.63% <92.63%> (ø)
pyomo/contrib/piecewise/__init__.py	100.00% <100.00%> (ø)
...ib/piecewise/transform/inner_representation_gdp.py	100.00% <100.00%> (+4.68%)	⬆️
...omo/contrib/piecewise/transform/multiple_choice.py	100.00% <100.00%> (ø)
...ib/piecewise/transform/outer_representation_gdp.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[bernalde]

Great PR. WIth the right framework of GDP, "advanced" formulations such as the PWL one from VIelma become easily implementable. I found a couple places where I had questions, but overall this is really great :)

[emma58]

@jsiirola, this is ready to look at again.