Adding UnitSet object to manage definition of base and derived units

[andrewlee94]

Part of #669

Summary/Motivation:

As a start towards removing a number of "magic string constants" in the IDAES codebase, this PR introduces a new UnitSet object which defines the set of known quantities in IDAES as properties along with their define units of measurement. For backwards compatibility, UnitSets allow quantities to be accessed in a dict-like fashion using a string for the quantity name, but as this is rolled out we will begin replacing instances of this in the codebase with direct references to the properties on the UnitSet.

Changes proposed in this PR:

• Add new UnitSet object, and replaces existent _default_units and _derived_units dicts with this in properties_meta.
• Adds standalone tests for UnitSets.
• Updates tests which relied on the old dict format to use UnitSets instead.

Legal Acknowledgement

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1. I agree my contributions are submitted under the license terms described in the LICENSE.txt file at the top level of this directory.
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[andrewlee94]

I should also write some docs for the new UnitSet, but I think these don;t need to be too extensive as this is mostly an internal object that users won't see much.

[jsiirola]

A couple questions (I think we can make this implementation simpler)...

[codecov]

Codecov Report

Base: 69.98% // Head: 70.04% // Increases project coverage by +0.05% 🎉

Coverage data is based on head (098dac6) compared to base (5182626).
Patch coverage: 98.55% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #991      +/-   ##
==========================================
+ Coverage   69.98%   70.04%   +0.05%     
==========================================
  Files         400      400              
  Lines       66171    66275     +104     
  Branches    12204    12201       -3     
==========================================
+ Hits        46308    46420     +112     
+ Misses      17486    17484       -2     
+ Partials     2377     2371       -6     

Impacted Files	Coverage Δ
idaes/core/base/property_meta.py	96.82% <98.52%> (+6.34%)	⬆️
idaes/core/base/reaction_base.py	82.19% <100.00%> (-0.10%)	⬇️
idaes/ver.py	61.53% <0.00%> (-4.62%)	⬇️
...roperties/general_helmholtz/helmholtz_functions.py	91.67% <0.00%> (+0.26%)	⬆️
idaes/core/dmf/util.py	55.55% <0.00%> (+1.19%)	⬆️
idaes/core/dmf/codesearch.py	100.00% <0.00%> (+1.44%)	⬆️
idaes/conftest.py	82.92% <0.00%> (+4.87%)	⬆️

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[andrewlee94]

@dangunter @jsiirola I think this is ready for the next round of reviews, and is hopefully almost ready to merge (I'll save updating the rest of the code base for a separate PR).