Attic'd interface.py.

Interfaces are defined by volumes in volume.py; ensembles for the interfaces
are defined in ensemble.py.
Ensemble.locate => Ensemble.find_valid_slices
Changes to sandbox. But sandbox appears broken...

It looks like some change to the storage broke the sandbox. It would have
been broken anyway before I made the imports absolute in this commit, so I'm
not going to try to fix it with this PR.