investigate using a monorepo for the new ArviZ

[amaloney]

for all of arviz-[base|plots|stats]

[ahartikainen]

What does this mean?

[amaloney]

Specifically it means putting arviz-base, arviz-plots, and arviz-stats into a single repo, but have each package installable on its own if a user needs/wants. Similar to how Jupyterlab has its packages folder, or how Bokeh has its python tool in the same repo as its js tool.