Adding another readable unit m^3

[itnonaka]

Changes proposed in this pull request

• Added the "m³" line in src/base/Units.cpp so Cantera can not only get m^3 but also read m³ as volume unit.
• Added the "m³" in unit basic conversion in test/general/test_units.cpp so it can also be tested.

If applicable, provide an example illustrating new features this pull request is introducing

When reading a .yaml file, it must have the units that the coefficients are referring to. However, sometimes people may use the same unit but in a different format, for example, m³ instead of m^3. Now, both ways can be read.

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows [contributing guidelines]
• The pull request is ready for review

[ischoegl]

Thanks, @itnonaka - this is a nice follow-up to the recently introduced support for the unicode alternative to m^2. I have a couple of minor requests.

• Documentation: search for instances of m^3 in *.md files ... there are several instances where m^3 is used in sample code. Replace 2 or 3 instances of sample code while leaving the rest as m^3 so users see both versions; however, replace all instances of m^3 that are part of comments.
• Documentation (cont'd): search for "Supported compound units are:" and add m³ to the Volume entry, so it's clear that it's an alternative to m^3 that is supported (you could also add the unicode version of dyn/cm^2 on the line above
• Python unit tests: add alternative lines in test_utils.py (as in the C++ google tests, add new lines with cm³ without replacing cm^3).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.43%. Comparing base (99550d0) to head (3678fa1).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1989      +/-   ##
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- Coverage   75.58%   75.43%   -0.15%     
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  Files         451      454       +3     
  Lines       56362    56348      -14     
  Branches     9299     9299              
==========================================
- Hits        42602    42508      -94     
- Misses      10616    10698      +82     
+ Partials     3144     3142       -2     

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