Py-ART version 1.8.0

Version 1.8.0 is the result of 5 months of work by 9 contributors.
Thanks to all contributors, especially those who have made their first
contribution to Py-ART.

Highlights of this release:

* Reading for ARM sonde files.
* Improvements to the GAMIC reader.
* Support for plotting radar data on maps using Cartopy.
* Hydrometeor classification routine.
* QPE routines.
* Support for Python 3.6.
* Various bug fixes and minor enhancements.

Contributors to Py-ART 1.8.0:

Jonathan J. Helmus
Jordi Figueras
Julia Signell
Kai Muehlbauer
Kirk North
Martin Rugna
Norm Wood
Scott Collis
Tulipa Silva

Nothing was deprecated by this version.