Create dataloader

[YoniSchirris]

• Allow loading of (offline available) different datasets
• Allow for different metadata to be loaded related to these datasets
• Allow for getting all data at once for any regression to be done
• 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

[LittlePea13]

Added a PR with Dataloader and Dataset. Unfortunately I could not improve performance much, each video preprocessing takes around 3 minutes, since it needs to do operations per frame to avoid memory issues. I someone knows more about Threading, I added a version that does the reading from the file on a different Thread but I do not know enough about threading and the performance was similar.

[MalcolmMielle]

I have made python functions in the utils.py file to transform a frame into a PPG signal. Should I move that to a folder containing common functions?