Function two_compartments_with_absorption 

pub fn two_compartments_with_absorption(
    x: &NalgebraVec<f64>,
    p: &NalgebraVec<f64>,
    t: f64,
    rateiv: NalgebraVec<f64>,
    _cov: &Covariates,
) -> NalgebraVec<f64>

Analytical solution for two compartment model with first-order absorption.

Assumptions

• p is a vector of length 4 with ke, ka, kcp and kpc in that order
• rateiv is a vector of length 1 with the value of the infusion rate (only one drug)
• x is a vector of length 3
• covariates are not used