Create dataloader

[YoniSchirris]

• [x] Allow loading of (offline available) different datasets
• [ ] Allow for different metadata to be loaded related to these datasets
• [x] Allow for getting all data at once for any regression to be done
• [x] On initializing the dataset, transform images to three PPGs (1 for each channel)
• [ ] Allow for data augmentation. E.g. using overlapping boxes of a long video