Gets the state variables.
The getState method is invoked by an ODESolver to obtain the initial state of the system.
The ODE solver advances the solution and then copies new values into the
state array at the end of the solution step.
state the state
void getRate(double state,
Gets the rate of change using the argument's state variables.
This method may be invoked many times with different intermediate states
as an ODESolver is carrying out the solution.