Integrating the HEOM

PHI implements two method to integrate the HEOM: Runga-Kutta 4'th (RK4) order integration and Runga-Kutta-Fehlberg 4/5 (RKF45) adaptive integration. In case of RK4 integration, only a fixed timestep $dt$ needs to be chosen. In case of RKF45 integration, a minimum timestep $dt_{min}$ and integration tolerance $\delta_I$ needs to be chosen.