Runtime type safety for Python bindings

[keenon]

Need to show an informative error when you call getXxJacobian() with anything other than a world (like a robot) Bonus: make an overloaded version of each getXxJacobian() that works with robots instead of worlds

This grows out of Yannis attempting to call getXxJacobian(robot), hoping to get a Jacobian for just that robot. It's a totally intuitive thing to do that you would expect to work, but instead it segfaults. We should print more helpful error messages when this happens.