Automatically download CORe50 instead of requiring user to download it.

[AndreaCossu]

Unlike other datasets like MNIST, CORe50 must be explicitly downloaded from the user. It would be better to have the same interface for all datasets.
Moreover, this behavior also breaks the script examples/simple_core50.py, since the provided data folder does not exist.

[AndreaCossu]

@vlomonaco can you confirm that the only files needed for CORe50 are the ones listed in the data loader description? Namely: core50_128x128, paths.pkl, LUP.pkl, labels.pkl, core50_imgs.npz. I assume that core50_128x128 refers to the unzipped folder, not to the zip file.

[vlomonaco]

yes, I think so! I think the preload of the dataset in ram should be disabled by default... so core50_imgs.npz is not really needed!

[AndreaCossu]

Ok, I will download also that file, just in case some user wants to use the preload option.