Add temperature dependence of rotational relaxation

[speth]

The temperature dependence of the rotational relaxation is given by Equation 12.112 of Chemically Reacting Flow (1st ed.). This factor results in an increase in mixture-averaged thermal conductivity of ~1% or less, and a similar increase in laminar flame speeds, at least for some test cases.

Originally reported here: https://groups.google.com/forum/#!topic/cantera-users/2_u6PjBqPcY

[codecov]

Codecov Report

Merging #547 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #547      +/-   ##
==========================================
+ Coverage   64.85%   64.86%   +<.01%     
==========================================
  Files         386      386              
  Lines       40865    40871       +6     
==========================================
+ Hits        26505    26511       +6     
  Misses      14360    14360

Impacted Files	Coverage Δ
include/cantera/transport/GasTransport.h	100% <ø> (ø)	⬆️
src/transport/GasTransport.cpp	88.82% <100%> (+0.11%)	⬆️
src/oneD/IonFlow.cpp	74.14% <100%> (+0.17%)	⬆️

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[bryanwweber]

The equation is 11.115 in the 2nd edition of Chemically Reacting Flow. Can you add a comment to that effect (maybe including both equation numbers) in the code?

[speth]

If we're going to start reliably referencing equations, I think we need a convenient shorthand, and a consistent place to put the full citations. As a starting point, I'll suggest just writing [Kee2017] Eq. 11.115 in the code comments, and providing the full citation, e.g.

[Kee2017] R. J. Kee, M. E. Coltrin, P. Glarborg, and H. Zhu. Chemically Reacting Flow: Theory and Practice. 2nd Ed. John Wiley and Sons, 2017.

in either the class docstring (i.e. the MixTransport class, in this case), a consolidated bibliography somewhere in the docs, or both.

[bryanwweber]

For now, I think this format is fine, with the bibliography going in the class docstring. Doxygen does have a \cite command that we might be able to make use of. It requires bibtex to be in the PATH, so maybe not worth it and we can just develop our own standard.