Callbacks: Generalize `yield_callbacks` into processing and output callbacks

Currently, net._yield_callbacks discerns between PrintLog and other callbacks with the effect that PrintLog is added last to the list of callbacks so it has access to all processed values added by other callbacks. Maybe we should generalize this by classifying callbacks into two groups: processing callbacks and output callbacks.

Output callbacks (identified by inheriting from an abstract subclass of Callback) are by default appended to the end of the callback list. This would pave the way for other output callbacks besides PrintLog such as TensorBoard logging callbacks.

Maybe PostponedCallback? PostAggregation?