Midpoint method: a second-order Runge-Kutta step from two slope evaluations (Q1676)

k_{1}=f(x_{n},y_{n}),\\qquad k_{2}=f\\bigl(x_{n}+\\tfrac{h}{2},\\,y_{n}+\\tfrac{h}{2}k_{1}\\bigr),\\qquad y_{n+1}=y_{n}+h\\,k_{2}