Add Tail ESS

[roualdes]

Summary:

Tail effective sample size (ESS) is one of the latest convergence diagnostics. While the packages RStan, posterior, and ArviZ use this new calculation, Stan/CmdStan does not yet.

Tail ESS should build off of Stan's current split chains functionality, which is available re Issue #2752.

Additional Information:

As another reference, here is RStan's implementation: https://github.com/stan-dev/rstan/blob/develop/rstan/rstan/R/monitor.R