Added custom view converter for datasets.norb.SmallNORB

We can't use a the DefaultViewConverter with SmallNORB, because it tries to fit a stereo frame into a single Conv2DSpace, rather than a pair of Conv2DSpaces. This PR adds a StereoViewConverter class to datasets/norb.py that does the job. Also added scripts/datasets/step_through_small_norb.py that verifies that the view converter works.