Load the dataset using a .npy file

The script to load the dataset use much less memory and is much faster.

**** Ian you need to run a experiment that loads the dataset once in order to create the .npy file. I don't have write access to your folder. Once it's done it will be fine for everyone. Do it from a computer with at least 4Go of memory. You also need to use the scrip make_submission_keypoints.py to generate the npy test file. (Might as well submit at this point ;-))

Vincent