Fix issues with Quantity class

[speth]

Changes proposed in this pull request

• Require pressure to be equal when mixing Quantity objects at constant enthalpy and pressure
• Use the phase-specific "native" state instead of defaulting to temperature, density, and mass fractions
• Fix addition of Quantity objects when the underlying Solution is set to use a molar property basis
• Implement set_equivalence_ratio and set_mixture_fraction methods
• Eliminate set_unnormalized_mass_fractions and set_unnormalized_mole_fractions methods that cannot be robustly implemented.

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

Closes #1461
Closes #1493
Closes #1446

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

Merging #1517 (53a0667) into main (36faa94) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1517      +/-   ##
==========================================
+ Coverage   70.42%   70.44%   +0.02%     
==========================================
  Files         375      375              
  Lines       58440    58462      +22     
  Branches    20919    20924       +5     
==========================================
+ Hits        41155    41185      +30     
+ Misses      14262    14254       -8     
  Partials     3023     3023              

Impacted Files	Coverage Δ
interfaces/cython/cantera/composite.py	84.68% <100.00%> (+0.63%)	⬆️

... and 1 file with indirect coverage changes

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[ischoegl]

Thanks … changes all look good to me.