Data pipelines

[lewisacidic]

The base Converter class should have responsibilities for filtering, standardising and producing splits removed from it. These should be implemented individually for datasets.

That said, a default pipeline should be provided, that will be used for all the datasets at the moment, but it may not be appropriate for all datasets!

[lewisacidic]

Completed a couple commits ago. The data module will need a rethink though.