More progress in both the world and the animal.  I'm having difficulty
keeping the interfaces separate, mostly because it's the easiest to just
make the classes have full access to one another.  I know, it's a terrible
practice, but I mostly want to get a working prototype whipped up for now.
Python really lets you do a lot of things you really shouldn't, because
you can (and people say C++ is bad when it comes that!).