Test of module runge_kutta_m integrates - dy_1/dtau = y --> y_1(tau) = exp(tau) - dy_2/dtau = 2tauy --> y_2(tau) = exp(tau^2) until tau = 2 Analytic result of y_1(2) = exp(2) ~ 7.38905609893 Analytic result of y_2(2) = exp(4) ~ 54.5981500331