Computations with stability functions can be very slow

[feipaat]

Convert the stability functions' object format to rational and floating point format in stability_function.py. Add warning comments in runge_kutta_method.py.

[ketch]

I believe that the issue we discussed today is resolved by doing

from nodepy import rk
rkm = rk.SSPRK3(16)
rkm.imaginary_stability_interval(mode='float')

Nevertheless, this needs to be documented better, so I'll leave the issue open until the documentation is improved.