Formulation of KPR

[Steven-Roberts]

There are 2 different parameterizations of KPR

1. Original paper: https://doi.org/10.1007/s10915-012-9662-z
2. Seeming more popular form: https://doi.org/10.1137/18M1205492

Need to settle on one and document. The original paper has a couple presets that are worth adding.