pydata/xarray: v0.9.0

This major release includes five months worth of enhancements and bug fixes from 24 contributors, including some significant changes that are not fully backwards compatible. Highlights include: Coordinates are now optional in the xarray data model, even for dimensions. Changes to caching, lazy loading and pickling to improve xarray's experience for parallel computing. Improvements for accessing and manipulating pandas.MultiIndex levels. Many new methods and functions, including quantile(), cumsum(), cumprod() combine_first set_index(), reset_index(), reorder_levels(), full_like(), zeros_like(), ones_like() open_dataarray(), compute(), Dataset.info(), testing.assert_equal(), testing.assert_identical(), and testing.assert_allclose(). For more details, see what's new