Analytic one, two and three compartment linear pharmacokinetic solutions with their parameter gradients from 'stan' automatic differentiation (Carpenter et al (2015) <doi:10.48550/arXiv.1509.07164>), eigen decompositions and derived-parameter conversions used by 'rxode2' (Wang, Hallow and James (2016) <doi:10.1002/psp4.12052>). Split out of 'rxode2' so its installation does not compile 'stan' AD items which made it take too long to compile by itself. The closed-form solutions follow the idea of the 'wnl' package (Bae, <https://CRAN.R-project.org/package=wnl>), though the implementation here is different.