Add model selection tools

[cpfiffer]

MCMCChains could use some model selection tools, like

• [x] Leave-one-out cross-validation
• [ ] BIC
• [ ] WAIC

Further recommendations are welcome, I'll add them to the list as they arrive.

[goedman]

Of interest on LOO etc might be http://www.jmlr.org/papers/volume11/watanabe10a/watanabe10a.pdf .

[torfjelde]

In the mean-time: https://arviz-devs.github.io/ArviZ.jl/dev/quickstart/#Additional-information-in-Turing.jl

[ParadaCarleton]

Implemented LOO-CV here. Feel free to import and reexport it.

[luiarthur]

Also related to LOO CV is Conditional Predictive Ordinates (CPO). See slides 19 - 24 in the link below:

http://webpages.math.luc.edu/~ebalderama/myfiles/modelchecking101_pres.pdf