Improve conversions between Pyomo and Sympy expressions

[jsiirola]

Fixes #2802, fixes #2803.

Summary/Motivation:

This PR improves support for converting Pyomo expressions to/from Sympy expressions. In particular:

• Remove the redundant code from cnf_walker: the CNF walker now builds on the general walker from sympy_tools (eliminates a significant amount of repeated code)
• Generate an exception when attempting to convert a Pyomo expression with an ExternalFunction expression node into a Sympy expression (fixes Sympy expressions leaking out of differentiation code #2803)
• Remove use of private sympy attributes
• Improve handling of NPV expressions

This also improves error checking in the core.logical_to_linear transformation. In particular, attempts to transform LogicalConstraints containing relational expressions will generate an exception (fixes #2802).

Changes proposed in this PR:

• (see above)

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

Patch coverage: 97.22% and project coverage change: +0.02 🎉

Comparison is base (96b408d) 87.01% compared to head (a1d2cb5) 87.04%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2806      +/-   ##
==========================================
+ Coverage   87.01%   87.04%   +0.02%     
==========================================
  Files         763      763              
  Lines       87276    87312      +36     
==========================================
+ Hits        75943    75998      +55     
+ Misses      11333    11314      -19     

Flag	Coverage Δ
linux	84.03% <97.22%> (+<0.01%)	⬆️
osx	73.54% <97.22%> (+<0.01%)	⬆️
other	84.21% <86.11%> (+<0.01%)	⬆️
win	81.56% <97.22%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
pyomo/core/plugins/transform/logical_to_linear.py	94.97% <85.71%> (+0.71%)	⬆️
pyomo/core/expr/cnf_walker.py	96.36% <100.00%> (+6.44%)	⬆️
pyomo/core/expr/sympy_tools.py	98.98% <100.00%> (+0.03%)	⬆️

... and 3 files with indirect coverage changes

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