Collision detection: in some cases, organisms overlap.

[AymericG]

When I ported the code from .NET, I couldn't get the collision detection algorithm to work at all. So I decided to mix the main collision detection concept (break the movement into small incremental movement until we hit something) and reused the _cellOrganisms index (tells you which cells are busy).

The code works 99% but sometimes two organisms will be allowed to overlap when they shouldn't.