Sample data

This tries to address the second part of #44 (closed) by adding a add-sample-instance "subcommand".

I must admit that this turned out to be hackier than I expected... I think that we should make the initialization (in particular instance reading) part of Main reusable so that we can have a separate binary for this and similar purposes, but I guess that's something for #42 (closed).

The commits are on top of those of !59 (merged) and should be similarly rebasable.