Resolve errors in mapping ScalarVar to numpy ndarray

[jsiirola]

Fixes #3418 .

Summary/Motivation:

This resolves how we map ScalarVar (and other scalar numeric expression types) into numpy ndarray objects. With this change, Pyomo Var objects behave in a manner consistent with other python scalars (like float).

Changes proposed in this PR:

• implicit casting of a ScalarVar to numpy array should result in a 0d ndarray
• add testing for (current) expected numpy behavior
• update assertExpressionsEqual to allow for lists / ndarrays of expressions (and to compare list and 1d ndarray as being equal)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.69%. Comparing base (180924a) to head (05bdaf7).
Report is 18 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3423   +/-   ##
=======================================
  Coverage   88.69%   88.69%           
=======================================
  Files         882      882           
  Lines      100615   100623    +8     
=======================================
+ Hits        89237    89247   +10     
+ Misses      11378    11376    -2     

Flag	Coverage Δ
linux	86.15% <100.00%> (+<0.01%)	⬆️
osx	76.17% <100.00%> (+<0.01%)	⬆️
other	86.65% <100.00%> (+<0.01%)	⬆️
win	84.54% <100.00%> (+<0.01%)	⬆️

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