Rename `graphics3d` and `graphics2d` to ``nphysics_testbed2d` and `nphysics_testbed3d`.
Simplify the way testbeds are initialized.

Instead of using a lambda, there is a `Testbed` struct that takes a physical world, and inspects it
to create the proper graphical objects.

There is no need to call something like `graphics.add_object()`, this is done automatically by the
testbed structure.