sphere_rk4:
  MATLAB/Octave version 9.9.0.1467703 (R2020b)
  Solve sphere_ode using Runge Kutta 4.
  sphere_ode models motion on the surface of a sphere.
  Graphics saved as "sphere_rk4_plot.png"
  Graphics saved as "sphere_rk4_plot3d.png"
  Graphics saved as "sphere_rk4_conserved.png"