Add PSIS-LFO CV

[devmotion]

While it's possible to perform PSIS-LFO CV manually currently, I think it would be more convenient if there would be first-class support for it in PosteriorStats. Admittedly, I haven't thought about potential design (issues) in detail yet.

Ref: arviz-devs/arviz-stats#177

[sethaxen]

I agree, this would be great to have. I haven't actually ever had an application with LFO-CV, so I'm not sure what interface would be most useful here. Do you have general thoughts?