Define an as_quantity() to evaluate a Pyomo expression to a pint Quantity

[jsiirola]

Fixes # .

Summary/Motivation:

This PR adds a as_quantity() function that will accept numeric-like objects (int / float / None / Var / Param / expression / etc) and return the value as a native pint Quantity (value + units).

This is useful for applications like user interfaces, and was requested by IDAES (@andrewlee94 and @dangunter)

Changes proposed in this PR:

• add as_quantity() function to evaluate an expression to a pint Quantity
• add tests (including expanded coverage of units_container)

Legal Acknowledgement

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1. I agree my contributions are submitted under the BSD license.
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[codecov]

Codecov Report

Merging #2222 (9016240) into main (0d35f56) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2222      +/-   ##
==========================================
+ Coverage   82.74%   82.77%   +0.02%     
==========================================
  Files         607      607              
  Lines       76792    76843      +51     
==========================================
+ Hits        63544    63608      +64     
+ Misses      13248    13235      -13     

Impacted Files	Coverage Δ
pyomo/core/base/units_container.py	98.03% <100.00%> (+2.28%)	⬆️
pyomo/environ/__init__.py	82.35% <100.00%> (ø)
pyomo/solvers/plugins/solvers/ASL.py	91.42% <0.00%> (+1.42%)	⬆️
pyomo/core/base/boolean_var.py	79.61% <0.00%> (+1.92%)	⬆️

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[mrmundt]

Just a comment, not a required change - should we standardize this at some point to change all instances that use dfs_postorder_stack to walk_expression?

[jsiirola]

Eventually? Yes. But that will require reworking the ExpressionValueVisitor to be based on the StreamBasedExpressionVisitor.

[carldlaird]

This looks reasonable to me.

[carldlaird]

Is this mostly for debugging, or is it used somewhere else?

[carldlaird]

I don't see how this works. If a Var has units that are themselves an expression (e.g., kg*m), then that expression won't have "_pint_unit"?

[carldlaird]

After discussions with @jsiirola I guess we create compound units instead of keeping the expression. Let's make sure there are tests that include Vars with units that are expressions.

[carldlaird]

This causes a broader question for the team. This PR is officially exposing pint from behind the Pyomo units. Do we want to do this?

I have no real concerns, however, we did quite a bit of work to make sure that pint was abstracted away (so we could replace pint with something else later if we wanted to). This decision may have been more about being unfamiliar with pint and not "ready to commit" to it. If we are ready to commit to pint, then there may be other simplifications we can make.

Just thoughts for discussion.

[blnicho]

@carldlaird we discussed your question at the developers meeting this week and the consensus was that we didn't have any concerns with exposing more of pint. Given the currently available alternatives to pint we think the likelihood of replacing it with something else is pretty slim.