Convert double absorption to sequential delays
Source:R/addSecondAbsorption.R
convertAbsSequential.RdReparameterizes a double-absorption model so the second path's delay is expressed relative to the first path's zero-order duration (`Tlag2 = Tk01`), matching Monolix's "sequential" double-absorption models. Only valid when the first path absorbs zero-order; raises an error otherwise.