Bear-Fish Simulation

Authors:

- Original idea: Bradley N. Miller et al., Python Programming In Context,  2nd Edition, Jones and Bartlett Learning, 2014.
- Modified and cleaned up by Prof. Victor Norman
- Cleaned up by Prof. Ken Arnold, Fall 2021

Files:

- bearfishsimulation.py: main simulation loop
- world.py: defines a grid World
- fish.py: defines Fish organisms that live in a World. Fish can 
  breed and suffer from overcrowding.
- bear.py: defines Bear organisms that live in a World and eat Fish.
