The predator and prey stepper

You will write one small step of the Lotka-Volterra equations and use it to follow the prey and the predators.

The idea. In a small time $dt$, the prey change by $(ax - bxy)\,dt$ and the predators by $(cxy - dy)\,dt$. This is a model for A-Level and IB.

Your tasks.

1. Finish step. It gives the new prey and the new predators after a time dt.
2. Finish run. It repeats the step n times, and gives the last prey and the last predators.
3. Set peak_year to the time at which the predators are most numerous in the first $10$ years.