NLv2: fix 0 operands in pow expressions

[jsiirola]

Fixes IDAES/idaes-pse#1084.

Summary/Motivation:

IDAES noted an error in the NLv2 writer for the case of pow expressions where the second operand was a constant 0. This resolves that error (adding a test). In addition, this PR adds logic to simplify expressions that are **0 and **1 (also adding tests).

Changes proposed in this PR:

• resolve error for binary operators with constant 0 operands
• add special-case simplification of **0 and **1 expressions

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[codecov]

Codecov Report

Base: 86.83% // Head: 86.97% // Increases project coverage by +0.14% 🎉

Coverage data is based on head (8bd1a90) compared to base (8fe21bb).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2726      +/-   ##
==========================================
+ Coverage   86.83%   86.97%   +0.14%     
==========================================
  Files         771      771              
  Lines       86162    86167       +5     
==========================================
+ Hits        74818    74948     +130     
+ Misses      11344    11219     -125     

Flag	Coverage Δ
linux	84.14% <100.00%> (+<0.01%)	⬆️
osx	73.59% <100.00%> (+<0.01%)	⬆️
other	84.32% <100.00%> (+0.20%)	⬆️
win	81.51% <100.00%> (?)

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

Impacted Files	Coverage Δ
pyomo/repn/plugins/nl_writer.py	93.60% <100.00%> (-0.14%)	⬇️
pyomo/util/subsystems.py	89.13% <0.00%> (-8.70%)	⬇️
...ontrib/appsi/utils/collect_vars_and_named_exprs.py	93.75% <0.00%> (-6.25%)	⬇️
pyomo/core/base/external.py	95.43% <0.00%> (-3.16%)	⬇️
pyomo/contrib/appsi/writers/nl_writer.py	84.39% <0.00%> (-2.32%)	⬇️
pyomo/repn/plugins/ampl/ampl_.py	81.40% <0.00%> (-1.86%)	⬇️
pyomo/core/expr/compare.py	96.52% <0.00%> (-1.74%)	⬇️
pyomo/contrib/appsi/solvers/ipopt.py	88.51% <0.00%> (-0.34%)	⬇️
pyomo/contrib/appsi/base.py	84.94% <0.00%> (-0.26%)	⬇️
... and 15 more

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