Simplify expressions generated by TemplateSumExpression

[jsiirola]

Fixes # .

Summary/Motivation:

This PR improves the automatic generation / simplification of expressions generated from TemplateSumExpression. In particular it:

• uses the mutable_expression context manager to generate the sum expression (so that expression simplification can occur)
• updates the mutable expression handlers to simplify sequential native numeric type terms into a single term
• updates the TemplateSumExpression resolver to return 0 or a single term instead of a sum expression when appropriate

This generates more natural (and flatter) sum expressions.

Changes proposed in this PR:

• collapse sequential native constants in the mutable expression context
• use the mutable expression context manager to resolve TemplateSumExpression objects

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

Attention: Patch coverage is 93.10345% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 88.39%. Comparing base (cd9d6ce) to head (ba105ea).

Files	Patch %	Lines
pyomo/core/expr/numeric_expr.py	93.33%	1 Missing ⚠️
pyomo/core/expr/template_expr.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3196      +/-   ##
==========================================
- Coverage   88.40%   88.39%   -0.02%     
==========================================
  Files         846      846              
  Lines       95131    95139       +8     
==========================================
- Hits        84099    84096       -3     
- Misses      11032    11043      +11     

Flag	Coverage Δ
linux	86.33% <93.10%> (-0.01%)	⬇️
osx	76.17% <93.10%> (-0.01%)	⬇️
other	86.52% <93.10%> (-0.02%)	⬇️
win	83.82% <93.10%> (-0.02%)	⬇️

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