'piecewise-Gibbs' missing from Science section

[ischoegl]

Reviewing species thermo models, it appears that piecewise-Gibbs is missing from the Science section, although it is documented in the C++ Mu0Poly docstring as well as the YAML Input File Reference.

"Science" documentation for other species thermo models is redundant, as corresponding equations also appear in docstrings, see NasaPoly1, Nasa9Poly1, Shomate and constant-cp (together with 2 temperature versions). I'd also like to note that variations in naming make it quite hard to locate C++ classes that handle these models; there is no link from science docs to the actual implementation.

Separate from this, there is also some doxygen documentation.

I opened this issue mainly to document a case in the context of Cantera/enhancements#178. The situation illustrates the conundrum of where to stick the documentation quite well. I do not think that it makes sense to fix it without considering the greater context.