Hunger and Reproduction

This introduces two new systems:

1. Hunger
2. Reproduction

Hunger adds a food counter on animals. They can increment this counter by eating (foxes eat rabbits, rabbits eat grass when undisturbed). If it ever drops to 0 or below the animal dies.

Reproduction allows animals with high food levels and a valid nearby tile to produce a baby.

All this is getting us closer to the natural boom and bust cycle we are trying to model. Foxes initially boom as they eat a lot of the initial rabbits, then they start starving. As the foxes die out, the rabbits start booming. Since foxes move in a deterministic manner when on the hunt (towards the upper right), they tend to gather in the upper right corner and starve even when there are plenty of rabbits left on the map.

I've also started thinking about how energy moves through the system. All animals burn energy to stay alive, meaning that energy is constantly exiting the system. Energy enters the system whenever rabbits eat since grass is an infinite resource. Energy can be both be added and removed from the system when a rabbit is eaten since foxes gain a fixed energy amount no matter how much energy the rabbit has stored. I'd love to track energy in the system more closely. Perhaps in a future release I might add some graphs.