Suggested order of tutorials

[rijobro]

I (and so maybe other new users) would find it really useful if there was a suggested order to some of the tutorials, building in complexity, rather than just alphabetical order.

[wyli]

agreed, pytorch tutorials use these categories

[rijobro]

As well as categories, it might be a good idea to start with e.g., nifti_read_example.ipynb and 3d_image_transforms.ipynb to get a feel for some building blocks before moving into whole networks.

[wyli]

the codebase is now restructured, I'm closing this ticket, please open new ones for further issues