Fix surface density/concentration calculation for multi-site species

[speth]

Changes proposed in this pull request

• Revise calculation of density for surface phases to correctly account for species that occupy multiple sites. This affects the concentrations (calculated in Phase as $C_k = \rho Y_k / W_k$) which in turn affects reaction rate calculations.

This resolves a regression introduced in #1350 that affects Cantera 3.0 and 3.1. The effect is most noticeable for phases where major species occupy multiple sites, such as in SiF4_NH3_mec.yaml where all the species have multiple occupancy.

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

• Partially 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

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 75.47%. Comparing base (5e351a1) to head (bb95641).
⚠️ Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2016      +/-   ##
==========================================
+ Coverage   75.43%   75.47%   +0.04%     
==========================================
  Files         454      454              
  Lines       56773    56804      +31     
  Branches     9345     9357      +12     
==========================================
+ Hits        42825    42872      +47     
+ Misses      10785    10764      -21     
- Partials     3163     3168       +5     

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[ischoegl]

Mentioning Cantera/enhancements#154, although I am unsure whether this is a tangent.

[speth]

I don't think that enhancement idea is relevant to the issue here. For phases, the state needs to be an intensive property, so I think the composition always needs to be in terms of relative amounts of one sort or another. The challenge here comes from the fact that there are three different conventions available for expressing this in the case of surface phases.

[ischoegl]

Thanks, @speth. Based on my reading of test_multi_site_density, this looks good to me!