Reviewed DataModule design+docs+types

In this MR, I try to review the use of the data module and go further in simplifications for the whole code:

• Slightly streamlined the datamodule approach
• Added documentation
• Added type annotations
• Added some TODOs for further discussion

Closes #7 (closed).

TODO:

• (@dcarron) Implement/document/type hint normaliser (ensure it is used correctly through all the models; integrate on the train script) and that train CLI works!
• (@andre.anjos) Deal with import reweight_BCEWithLogitsLoss