Add ml cryo from ioSPI

[ninamiolane]

This PR adds io functionalities related to machine learning datasets to reduceSPI, instead of having them in ioSPI.

It also adds an ignore rule for I202 in flake8, since isort and flake8 are conflicting there.

This is because the methods of this file are:

• very specific to machine learning models (split in train test for example),
• also specific to our own organization of the files when run on VAEGAN.

Therefore, they are not self-consistent, and should not be in ioSPI.