Support saving and loading of replicated and additive tensors

[mortendahl]

There is no inherent reason why you can't do this, in fact it might be useful to persist data in an encrypted (secret shared) state.

Some design is needed to get this to work, and it might be good to get that sorted sooner rather than later to make sure we don't move on with a false assumption.

[mortendahl]

@rdragos this might be something for you to spending a bit of time thinking about next week