Conserve sum of mass fractions in FlowReactor

[speth]

Changes proposed in this pull request

• Modify species equations in FlowReactor to impose a constraint on the sum of the mass fractions (or specifically, that $\frac{d}{dt} \sum Y_k = 0$
• Fix some plotting units in 1D_pfr_surfchem.py example
• Add a regression test for this example based on the results in the original Sandia report

If applicable, fill in the issue number this pull request is fixing

• Resolves plug flow reactor: silicon nitride deposition example: sdot and pressure increase issues #1996

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

Updated plot from 1D_pfr_surfchem.py

Note that the scale of variation of the pressure (green line) is ~0.01 Pa.

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.52%. Comparing base (fd4d005) to head (7576c7a).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2018   +/-   ##
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  Coverage   75.51%   75.52%           
=======================================
  Files         455      455           
  Lines       56932    56937    +5     
  Branches     9362     9363    +1     
=======================================
+ Hits        42994    42999    +5     
  Misses      10765    10765           
  Partials     3173     3173           

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[ischoegl]

Looks good to me!