predator_midpoint  Solve predator-prey equation with midpoint method
  Plot the solution.

  Input value N not supplied.  Using N = 400
  Graphics saved as "predator_midpoint.png"
  Graphics saved as "predator_midpoint_phase.png"
  Graphics saved as "predator_midpoint_conserved.png"
  h(1) = 3.22741, h(n+1) = 3.25813