E + S <--> ES | kf=0.003, kr=0.001 | E=100, S=50, ES=0
ES --> E + P | kf=0.002 | P=0
@obs Free_enzyme: u[E]
@obs Total_enzyme: u[E] + u[ES]
@obs Product: u[P]
@sim tspan: [0, 100]
