The Simulation class has been modified to make the fish act on one step 
then have the PondStocker adjust the population on the next. In this way it
is easy to see the fish added by PondStocker without confusing them with
fish created by breeding.

To see this better, you can change the probability of dying to be higher or of
breeding to be lower, so the population tends to decrease and needs frequent
restocking.

Substitute the Simulation class given here in for the original simulation class 
and add the PondStocker class, and this code will run with the other MBS code.

