Feature: Support multiple RK45 Butcher tableaus (Dormand-Prince, Fehlberg, Cash-Karp)

[sablabium]

Why are we doing this?

Currently, our rk45 solver implementation in ODE_solver.cpp has hardcoded Dormand-Prince coefficients inside the main loop. While Dormand-Prince is a great default, we already have an enum defined for other popular adaptive methods:

enum step_method {
    fehlberg,
    dormand_prince,
    Cash_Karp,
};

However, selecting fehlberg or Cash_Karp currently has no effect on the integration.