Py-ART Version 1.9.0 Picasso

Version 1.9.0 Picasso is the result of 12 months of work by 13 contributors.
Thanks to all contributors, especially those who have made their first contribution to Py-ART.

Highlights of this release:

* Allow graph titles and filenames to use time at beginning of sweep.
* Addition of a faster calculation of texture of the velocity field.
* Addition of a raster keyword to plotting functions.
* Addition of a alpha keyword to the PPI map plotting function.
* Contributors guide was added to help guide developers.
* Updated dealiasing code that corrects for over/under folded regions.
* Gate filter option has been added to VPT plots.
* Nearest neighbor weighting function was added as an option to griding.
* Two new KDP methods (Vulpiani et al., 2012) and (Schneebeli et al., 2014).
* Various bug fixes and minor enhancements.

Contributors to Py-ART 1.9.0 Picasso:

Chris Nardi - csnardi
Daniel Wolfensberger - wolfidan
Jonathan J. Helmus - jjhelmus
Jordi Figueras - meteoswiss-mdr
Mark Picel - mhpicel
Ramesh - goharkay
Robert Jackson - rcjackson
Scott Collis scollis
Sean Freeman - freemansw1
Steve Nesbitt - swnesbitt
Timothy Lang - tjlang
Valentin Louf - vlouf
Zachary Sherman - zssherman

Nothing was deprecated by this version.