Implement ODE schemes from SciPy for NEI solve? #26
Description
As brought up in #20, there are some existing ODE solvers in SciPy. We could potentially include more time advances using, for example, the Runge-Kutta 4th order method which has also been used for NEI analyses. The eigenvalue method should still remain the default. RK4 and other methods could be useful for testing purposes also.