Make namespace flat

Plan is to have a flat namespace for public functions:

• plot_ (plot_forest, plot_autocorrelation, etc)
• stat_ (not wild about this naming convention - maybe just make sure these names are descriptive and exposed in the top-level namespace. thoughts?)
• other functions (convert_to_xarray, style.use)
• Rename load_arviz_data to load_dataset
• Make sure libraries like matplotlib.pyplot or xarray are not making it into the arviz namespace.

[ColCarroll]

My only hesitation with that is that there will be different return types (turning a numpy array into inference data is a ton of guessing about intentions). What about exposing

to_inference_data
to_dataset
from_pymc3
from_stan

in the top level, and rename the directory to convert, so you can az.convert.dict_to_dataset or az.convert.numpy_to_dataarray if you need extra specificity.

I know it is not consistent, but I sort of like that from_pymc3 accepts a trace argument, while from_stan accepts a fit object, which will be familiar to users of the library. Hopefully from_pyro and from_tfp will show up soon!

[ColCarroll]

Marking this as done, by the way.