predator_euler  Solve predator-prey equation with Euler method
  Plot the solution.

  Input value N not supplied.  Using N = 100
  Graphics saved as "predator_euler_100.png"
  Graphics saved as "predator_euler_phase.png"
[Warning: Imaginary parts of complex X and/or Y arguments ignored.] 
[> In predator_euler (line 67)
In run (line 91)] 
  Graphics saved as "predator_euler_conserved.png"