Changes applied to PyArt to process airborne radar data,
specifically used NOAA P-3 Doppler radar data converted
using the NCAR Radx software package.
Merge remote-tracking branch 'arm_doe/master' into airborne

Allows the reading and displaying of cfradial airborne radar data.
Updated to test working order with lower fuselage data
ENH: extra volume preparation

Adding the ability to remove poor gates based on the normalized coherent
power and cross correlation ratio fields, assuming prep = 1. This is
added (quite easily) using an additional kwarg called extra_prep. This
should further help the 4DD algorithm by removing noise from the
receiver and second-trip echoes. I decided to add this feature as I
noticed the lowdbz and highdbz thresholds for the reflectivity field
were not adequately removing enough bad gates, especially those
representing second-trip echoes.
DOC: extra prep docs

Adding documentation necessary for the extra volume preparation inputs.
I am also adding in a check to make sure that the radar has the
necessary fields to perform the extra volume preparation, assuming
extra_prep is True.
Additional changes to Airborne RadarDisplay code to increase variables read in cfradial.

Added additional coordinate transformations.
Merge branch 'airborne'
BUG: Treat radar coordinates as gate centers in plots

The range, azimuth, elevation coordinates in radar files typically specify
the center of the gate, when plotting Py-ART uses these coordinates to
interpolate and extrapolate the gates edges which are then used to plot
the data.

Prior to this the centers were used to plot which resulted in the plots
missing the last range gate and the last ray.
Merge pull request #179 from jjhelmus/pcolor_bug

BUG: Treat radar coordinates as gate centers in plots

Thanks to @deeplycloudy for pointing out this bug
MAINT: NEXRAD archive file now returned with float32 fields
MAINT: added replace_existing parameter to add_field method.
Merge pull request #181 from jjhelmus/master

MAINT: float32 NEXRAD fields and replace_existing parameter
BUG: Fixed cfradial unit test to work with netCDF4 >= 1.1.1

See https://github.com/Unidata/netcdf4-python/pull/269 for change in
netCDF4 library.
Merge pull request #183 from jjhelmus/bug_netcdf_maskandscale

BUG: Fixed cfradial unit test to work with netCDF4 >= 1.1.1
MAINT: Sigmet extended headers can now be read.

This is in preparation for reading NOAA Hurrican Hunter Aircraft
tail radar data which has an extended Sigmet header.
STY: PEP8 and docstring clean of airborne code.
Adding arm_time_variables parameter to write_grid

I am adding the 'arm_time_variables' parameter to the write_grid
function in the grid_io module. This is consistent with the parameter of
the same name found in the cfradial module.
Adding arm_time_variables to write method

Similar to the previous commit, but this time adding the
arm_time_variables to the Grid object write method.
Checking PEP 8 standards...

All changed files now meet PEP 8 standards!
ENH: read_sigmet can read NOAA Hurrican hunter aircraft radar.