Implement compressed training and test on AutoPET

[Sllambias]

Allows saving as NPZ and NPY for preprocessed files REMOVES extension from splits. This is not backwards compatible.

• [x] Implement saving as npz
• [x] Implement loading as npz
• [x] Test training time for 3D
• [x] Test training time for 2D
• [x] Write in-code warnings and such
• [ ] Add docs in GitHub

[Sllambias]

@asbjrnmunk if you have any suggestions in general for this lmk here