The implementation now correspond to the definitions in the readily
available reference:

http://personalpages.manchester.ac.uk/staff/david.d.apsley/specturb.pdf

in which a large number of linear and non-linear models are presented in
a clean and consistent manner.  This has made re-implementation and
checking far easier than working from the original references which anyway
are no longer available for the LienCubicKE and ShihQuadraticKE models.