Settle on cleaner API for magic registration. The official API will be: - ip.register_magics(*args): for registering one or more classes or instances that subclass the main magic.Magics class. This will be the *only* method for registering magics that have a signature f(self, line,...). - ip.function_as_magic: for registering one-off magics made from a standalone function with the signatures f(line), f(line, cell) or f(line, cell=None). We will support, for backwards compatibility, the old ip.define_magic, but it will print a deprecation warning.