Add arviz integration

[sharanry]

Add traceplot() and plot_posterior() functions.

[junpenglao]

I dont think this is needed - we should use https://github.com/arviz-devs/arviz directly.

[sharanry]

@junpenglao I was unaware of this. So does arviz give direct support to ndarray and/or xarray?

[junpenglao]

it would be xarray native - keep an eye on the purpose structure for storing the result

[ColCarroll]

Yes - over the next week or so I plan to work on adding preliminary support for xarray traces in arviz, using the current, super-preliminary structure.

[twiecki]

pymc4 should also target an xarray backend that is natively compatible with arviz.