Adding a Callback which has the same name as an attribute no longer raises an exception

[jph00]

When you add, for example, a callback called Foo to a Learner, it makes that available in the foo attribute of the learner.

Previously, if you added a Foo callback to a learner that already had a foo attribute, that would raise an exception. However, since it's not possible to know what other callbacks exist and what attributes might be added to a learner, we've now removed the code that raises an exception.