Xarray Release 0.18.0

[Thomas Nicholas]

v0.18.0 of xarray is out!

This release brings a few important performance improvements, a wide range of
usability upgrades, lots of bug fixes, and some new features. These include
a plugin API to add backend engines, a new theme for the documentation,
curve fitting methods, and several new plotting functions.

Many thanks to the 38 contributors to this release: Aaron Spring, Alessandro Amici, Alex Marandon, Alistair Miles, Ana Paula Krelling, Anderson Banihirwe, Aureliana Barghini, Baudouin Raoult, Benoit Bovy, Blair Bonnett, David Trémouilles, Deepak Cherian, Gabriel Medeiros Abrahão, Giacomo Caria, Hauke Schulz, Illviljan, Mathias Hauser, Matthias Bussonnier, Mattia Almansi, Maximilian Roos, Ray Bell, Richard Kleijn, Ryan Abernathey, Sam Levang, Spencer Clark, Spencer Jones, Tammas Loughran, Tobias Kölling, Todd, Tom Nicholas, Tom White, Victor Negîrneac, Xianxiang Li, Zeb Nicholls, crusaderky, dschwoerer, johnomotani, keewis

Full release notes: http://xarray.pydata.org/en/stable/whats-new.html