Add momentum, support for Andre Holzapfel's dataset, fix bug backpropogating errors. Clean-up.

- Adds support for a momentum in training.
- Fix a bug backpropogating errors that was leading to poor,
non-converging training results.
- Changes to support Andre Holzapfel's dataset. Automatic onset
realignment now limited to only the al_ onsets. Stereo audio is now also
downmixed as necessary.
- Clean-up the namespaces of a bunch of the classes.
- It's now possible to schedule a number of different neural networks to
be trained with various configurations and have their performance logged
(on training, validation and test datasets). This aids emperical
selection of good parameters.