Support cython functions through Initializer()

[jsiirola]

Fixes #2374 .

Summary/Motivation:

This resolves issues where Initializer did not correctly identify all functions (notably cythonized functions). This updates the logic for processing initialization functions and expands the Initializer() unit tests.

Changes proposed in this PR:

• Correctly detect cythonized functions as callable functions
• update Initializer unit tests
• Fix issue in pyomo.core.__init__.py that was preventing the import of pyomo.core.util

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[codecov]

Codecov Report

Merging #2421 (afca676) into main (622cf52) will increase coverage by 0.00%.
The diff coverage is 88.88%.

@@           Coverage Diff           @@
##             main    #2421   +/-   ##
=======================================
  Coverage   85.89%   85.90%           
=======================================
  Files         617      617           
  Lines       76198    76204    +6     
=======================================
+ Hits        65453    65465   +12     
+ Misses      10745    10739    -6     

Flag	Coverage Δ
linux	82.64% <88.88%> (+<0.01%)	⬆️
osx	72.87% <88.88%> (+<0.01%)	⬆️
other	82.83% <88.88%> (+<0.01%)	⬆️
win	79.65% <88.88%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
pyomo/core/__init__.py	100.00% <ø> (ø)
pyomo/core/base/initializer.py	99.50% <88.88%> (+0.01%)	⬆️
examples/pyomobook/test_book_examples.py	96.80% <0.00%> (+0.79%)	⬆️
pyomo/opt/results/container.py	75.75% <0.00%> (+0.90%)	⬆️
pyomo/common/getGSL.py	66.66% <0.00%> (+3.33%)	⬆️

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[mrmundt]

Does one of the new tests check against this assumption?

[jsiirola]

Yes: test_initializer.py:531 (test_no_argspec)