Changes proposed in this pull request

• Implement serialization of Peng-Robinson phases
• Update Redlich-Kwong serialization to save critical parameters for species where those were provided instead of R-K specific parameters
• Some formatting cleanup of the PengRobinson class
• Fix incorrect acentric factors for O2 and N2 in critical-properties.yaml

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

import cantera as ct
p = ct.Solution('test/data/thermo-models.yaml', 'CO2-PR')
print(p.write_yaml(skip_user_defined=True))

...
species:
  - name: CO2
    composition: {C: 1.0, O: 2.0}
    thermo:
      model: NASA7
      temperature-ranges: [200.0, 1000.0, 3500.0]
      data:
        - [2.35677352, 8.98459677e-03, -7.12356269e-06, 2.45919022e-09,
        -1.43699548e-13, -4.83719697e+04, 9.90105222]
        - [3.85746029, 4.41437026e-03, -2.21481404e-06, 5.23490188e-10,
        -4.72084164e-14, -4.8759166e+04, 2.27163806]
    equation-of-state:
      model: Peng-Robinson
      a: 3.958134e+05
      b: 0.0266275
      acentric-factor: 0.228
      binary-a:
        H2O: 7.897e+06
...

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