Sampling enhancements Allow specification of a key with -k for doing sampling between a single datum and the rest of the database. (The implementation is slightly icky, as mentioned in the previous commit message; hopefully this situation will not remain for too long). While we're at it, allow -r/--resultlength as a synonym for --nsamples (if --nsamples is not given) in order better to reflect the analogy between querying and sampling.