2nd specialist week work

[jalabort]

This PR touches many areas of nucleus.

In particular, it addresses, to ceratin extend, the following issues:

• Revisit transform module #3 Revisits the transform module.
• Add keras applications backbones #6 Adds keras-applications backbones.
• Fix model saving issues #10 Fixes model saving issues.
• Replace export for public #9 Replaces export for public.
• Define loss functions #14 Defines a first version of the Yolo loss function.
• Create detection head layers #8 Creates a first version of the Yolo detection head.
• Commit relevant notebooks #4 Commits a first set of notebooks.

Most of these issues will remain open for the time being. Nonetheless a considerable amount of work was done in this PR to push them forward.

TODO

• Commit relevant notebooks.