[WIP] Add a QuadraticExpression node

[michaelbynum]

Fixes #1761, #230.

Summary/Motivation:

This PR adds a QuadraticExpression node to the expression system. This PR does not add any special handling of QuadraticExpression (that can be added later as needed).

This can reduce the construction time for quadratic constraints and it allows Pyomo-based algorithms to add special handling of quadratic constraints.

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[codecov]

Codecov Report

Merging #2329 (5c72529) into main (b48c886) will increase coverage by 0.00%.
The diff coverage is 97.19%.

@@           Coverage Diff            @@
##             main    #2329    +/-   ##
========================================
  Coverage   85.61%   85.62%            
========================================
  Files         618      618            
  Lines       76206    76313   +107     
========================================
+ Hits        65241    65340    +99     
- Misses      10965    10973     +8     

Flag	Coverage Δ
linux	82.44% <97.19%> (+0.02%)	⬆️
osx	72.70% <97.19%> (+0.03%)	⬆️
other	82.39% <97.19%> (+0.02%)	⬆️
win	79.49% <97.19%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
pyomo/core/expr/current.py	100.00% <ø> (ø)
pyomo/repn/standard_repn.py	88.54% <ø> (ø)
pyomo/core/expr/numeric_expr.py	98.16% <97.00%> (-0.13%)	⬇️
pyomo/core/expr/compare.py	98.87% <100.00%> (+0.09%)	⬆️
pyomo/common/env.py	89.15% <0.00%> (-2.01%)	⬇️

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[jsiirola]

I think a QuadraticExpression node can make a lot of sense. However, I wonder if this is the right data structure, or rather if the right answer is to store it as ?

[mrmundt]

This has been added to the Archived design discussions wiki; will close for now.