sphere_euler:
  MATLAB/Octave version 9.9.0.1467703 (R2020b)
  Solve sphere_ode using Euler.
  sphere_ode models motion on the surface of a sphere.
  Graphics saved as "sphere_euler_plot.png"
  Graphics saved as "sphere_euler_plot3d.png"
  Graphics saved as "sphere_euler_conserved.png"