ranked and split r-hat

[bob-carpenter]

Add implementation of ranked R-hat as described in this paper:

• Vehtari, A., Gelman, A., Simpson, D., Carpenter, B. and Bürkner, P.C., 2021. Rank-normalization, folding, and localization: An improved R ̂ for assessing convergence of MCMC (with discussion). Bayesian Analysis, 16(2), pp.667-718.