Rework handling of 'dimensionless' units in Pyomo

[jsiirola]

Fixes #1570.

Summary/Motivation:

@andrewlee94 reported strangeness with units where the following did not raise an error:

m = ConcreteModel()
m.v = Var(units=units.dimensionless)
m.v.set_value(42*units.s)

This was due to get_units() returning None instead of dimensionless. This caused Pyomo to not be able to distinguish between Var(), and Var(units=units.dimensionless). This PR changes the current behavior so that get_units() will always return a _PyomoUnit.

In addition, this PR reworks the PintUnitExtractionVisitor to make it slightly more performant.

While much of #1570 has already been fixed on main, this PR fixes the last known issues.

Changes proposed in this PR:

• dimentionless is returned from get_units() instead of None
• efficiency improvements in the PintUnitExtractionVisitor
• cleaned up some long lines and string formatting

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

Base: 87.13% // Head: 87.13% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (f2e3372) compared to base (7edd5f0).
Patch coverage: 90.24% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2691   +/-   ##
=======================================
  Coverage   87.13%   87.13%           
=======================================
  Files         757      757           
  Lines       84664    84674   +10     
=======================================
+ Hits        73770    73780   +10     
  Misses      10894    10894           

Flag	Coverage Δ
linux	84.55% <90.24%> (+<0.01%)	⬆️
osx	74.58% <90.24%> (+<0.01%)	⬆️
other	84.73% <90.24%> (+<0.01%)	⬆️
win	81.89% <90.24%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
pyomo/core/base/units_container.py	97.87% <90.24%> (+0.05%)	⬆️
pyomo/solvers/plugins/solvers/ASL.py	91.42% <0.00%> (-1.43%)	⬇️
pyomo/core/expr/visitor.py	93.40% <0.00%> (+0.15%)	⬆️
pyomo/util/check_units.py	91.46% <0.00%> (+1.21%)	⬆️

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[mrmundt]

I don't know this code well enough to have a good idea of whether this is an efficient/good solution, but for what I do know, it looks fine to me (minus the smallest of typos).