Simplify using rhat for single chain

[Martin Modrák]

The rhat diagnostic can be (with some loss of sensitivity) used also for single chains. It might thus be sensible to expose the Rhat with also a single chain signature, not just a vector.

[Matthew Parno]

This is a great idea. It is unclear to me how to compute rhat for a single chain in the Multilevel MCMC case, but I’ve added a simple Rhat function to the SampleCollection class, which is used for all of our other MCMC techniques. This is in commit 0887531 and will be merged soon.