DESCRIPTION: PK oral + indirect response model

input =  {Tlag, ka, V, Cl, Imax, IC50, kin, kout}

EQUATION:
Cc = pkmodel(Tlag, ka, V, Cl)
E_0 = kin/kout
ddt_E = kin*(1-Imax*Cc/(Cc+IC50)) - kout*E

OUTPUT:
output = {Cc, E}
