add encryption at rest

[ben-jones]

We should design our data so that it is encrypted at rest. This should be incorporated into the new backend/storage formats design. I'm thinking that we could include a bit of metadata outside the encrypted stuff, e.g. the country we measured in and what type of experiment this is. I believe the Toronto IRB protocol says that we will do encryption at rest (we currently use an encrypted disk), so we need to incorporate this into the next design.