Move to gsl_odeiv2_step_rk8pd.

Still 8 digits per Ganapol.
Solve can run to 13.4 while still advancing fp monatonically.
gsl_odeiv2_step_rk8pd doesn't need jac

Zero difference in output w/ or w/o jac, though
this likely means I've not been employing it thus far.
Clean up comments