Fix handling of POW in Baron writer

[jsiirola]

Fixes # .

Summary/Motivation:

@shermanjasonaf noted a bug in the Baron writer where expressions of the form "m.x ** (c + m.p * m.y)" where m.p is a mutable Param with value 0 would be written to the BAR file as "x ^ (3 + 0*y)", which is not legal (Baron sees the right argument as variable).

This PR resolves the issue by re-evaluating fixed arguments to resolve them to numeric constants, and adds tests exercising this edge case,

Changes proposed in this PR:

• Resolve fixed arguments to POW to numbers when generating Baron expressions

Legal Acknowledgement

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

Codecov Report

Base: 87.06% // Head: 87.06% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (b124d10) compared to base (553bc0b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2693      +/-   ##
==========================================
- Coverage   87.06%   87.06%   -0.01%     
==========================================
  Files         757      757              
  Lines       84585    84591       +6     
==========================================
+ Hits        73645    73647       +2     
- Misses      10940    10944       +4     

Flag	Coverage Δ
linux	84.45% <100.00%> (-0.01%)	⬇️
osx	?
other	84.63% <100.00%> (+<0.01%)	⬆️
win	81.79% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
pyomo/repn/plugins/baron_writer.py	81.84% <100.00%> (+0.27%)	⬆️
pyomo/contrib/mindtpy/iterate.py	79.54% <0.00%> (-1.82%)	⬇️
pyomo/common/download.py	92.17% <0.00%> (-0.44%)	⬇️
pyomo/core/expr/visitor.py	93.40% <0.00%> (+0.15%)	⬆️

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