Migrate "Science" docs

[speth]

Changes proposed in this pull request

• Migrate content from the cantera-website "Science" section into the versioned Sphinx documentation
• Significantly revise "species thermo" documentation
  • document all SpeciesThermoInterpType models (e.g. NASA polynomials), models derived from PDSS (e.g. temperature-dependent density), and models where the parameters are processed directly by the ThermoPhase model (e.g. Peng-Robinson).
  • Try to bridge the gap between the typical definition of "standard state" and Cantera's mixed usage of "reference state" and "standard state".
• Reorganize reactor network docs to split out a tutorial (now in the "User Guide" section) and a description of the CVODES inner workings (now in the "Develop" section).

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

Continues work on Cantera/enhancements#178
Closes Cantera/cantera-website#249

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

[speth]

Requesting @decaluwe's review specifically for the updates to the "species thermo" and "standard state" docs.

[decaluwe]

Thanks @speth --will get to it! Excited to see this come through (and wish I had done more to help get it there).

[bryanwweber]

Thanks for all this work @speth! I'm probably not going to have time to review this all again, so please feel free to incorporate or discard these comments at your discretion. If you have any questions, I'm happy to help clarify 😸

[decaluwe]

Thanks @speth for this massive undertaking. I've added a number of thoughts. The two most relevant:

1. The phase thermo sections have a lot less detail than the kinetics sections. I think my preference would be to get this merged before adding more on top, but if you'd rather add it in now (assuming you want to at all), lmk and I can help.
2. The reference vs. standard state. I added some thoughts to tune the wording. Long term, I think we can get rid of the non-standard use of standard state in RedlichKwong and PengRobinson, assuming I am correct that they are the only two that use it. In those two is it just a minor mathematical convenience and can be pretty easily removed/adjusted. That said I do like "reference state" better than "standard state," generally, since a little digging shows that there is no single "standard" that applies across fields. The user provided info really is just a reference: "We know the properties well at this one condition." But that is a different windmill for a different day, I suppose.

[ischoegl]

Thank you, @speth! All good from my side. As I didn’t do heavy lifting to review, it’s not my place to approve though.