Python 3 requires explicit allow_pickle for numpy saving

[Didou09]

For retro-compatibility with old python 2 saved files, and for better control ove the saved files, the parameter allow_pickle should be set by the user when loading a numpy file with tofu

This is a known difference between numpy for python 2 and python 3, for safety issues:
https://stackoverflow.com/questions/41696360/numpy-consequences-of-using-np-save-with-allow-pickle-false