Function midpoint_euler_step

* @brief Compute next step approximation using the midpoint-Euler * method. * @f[y_{n+1} = y_n + dx\, f\left(x_n+\frac{1}{2}dx, * y_n + \frac{1}{2}dx\,f\left(x_n,y_n\right)\right)@f] * @param[in] dx step size * @param[in,out] x take @f$x_n@f$ and compute @f$x_{n+1}@f$ * @param[in,out] y take @f$y_n@f$ and compute @f$y_{n+1}@f$ * @param[in,out] dy compute @f$y_n+\frac{1}{2}dx\,f\left(x_n