Rearrange data modules

[ColCarroll]

Fairly big reorganization. The major difference here is that the utils module is removed, in favor of a data module. Additionally, there is arviz.data.convert which contains all the functions for converting stuff into xarray and netcdf objects. This should let the conversion functions breathe a bit more (xarray_utils was pushing 1k lines).

Two followups might be:

• Rename plotting functions plot_trace, plot_autocorr, etc,
• Think about where the functions in plot_utils ought to go: xarray_var_iter is used everywhere, for example.

[ColCarroll]

(merged so no one has to work off a branch)