pep8
Make norb and norb_small dataset use the cache
Make the test use less memory.

It still use ~10G!
pep8
Make the dataset SmallNORB implement the stop parameter and use it in tests.

This lower this test memory usage from 12G to 1.6G
Merge pull request #883 from nouiz/datasets

Make the norb datasets tests use 1.5G instead of 12G