Add tpx citations

[ischoegl]

Changes proposed in this pull request

• Add doxygen references for tpx pure substances.
• Add doxygen references for FalloffRate specializations
• Add doxygen reference for NASA 9-coefficient polynomials

Checklist

• [x] The pull request includes a clear description of this code change
• [x] Commit messages have short titles and reference relevant issues
• [x] Build passes (scons build & scons test) and unit tests address code coverage
• [x] Style & formatting of contributed code follows contributing guidelines
• [x] The pull request is ready for review

[codecov[bot]]

Codecov Report

Merging #1575 (1807e03) into main (1f580a3) will increase coverage by 0.04%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1575      +/-   ##
==========================================
+ Coverage   70.49%   70.53%   +0.04%     
==========================================
  Files         379      379              
  Lines       59110    59110              
  Branches    21232    21232              
==========================================
+ Hits        41671    41695      +24     
+ Misses      14363    14340      -23     
+ Partials     3076     3075       -1     

Files Changed	Coverage Δ
include/cantera/kinetics/Falloff.h	81.63% <ø> (ø)
include/cantera/thermo/Nasa9Poly1.h	0.00% <ø> (ø)
src/tpx/CarbonDioxide.h	100.00% <ø> (ø)
src/tpx/HFC134a.h	100.00% <ø> (ø)
src/tpx/Heptane.h	100.00% <ø> (ø)
src/tpx/Hydrogen.h	100.00% <ø> (ø)
src/tpx/Methane.h	100.00% <ø> (ø)
src/tpx/Nitrogen.h	100.00% <ø> (ø)
src/tpx/Oxygen.h	100.00% <ø> (ø)
src/tpx/Water.h	100.00% <ø> (ø)

... and 4 files with indirect coverage changes

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