Deprecate generator method interface

[cboulay]

Due to shortcomings with the generator method interface, we've replaced all of the generator methods with normal Python classes. Note: these classes implement the generator interface (__iter__). We kept backwards compatibility by also providing tiny functions that return an instance of our classes.

I think we should provide a deprecation warning whenever these tiny generator functions are used.
Additionally, and more importantly, we should update all of our unit tests to use the new class (Transformer) interface.