This isn't really ever used in Nengo code as all of the operator step functions get a handle to the ndarray and copy into them that way. But, it's used in `test_simulator`, and was in the original `Accessor` class, so I believe this is mainly useful for debugging the simulator; i.e., it's not likely that users will ever encounter this. It wouldn't be useful for debugging to automatically create something, as then the simulator would continue going as if nothing happened. But yeah, it would be confusing the first time, so I'll add something to the docstring, and to the KeyError raised by `__getitem__`.