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'
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.
MAINT: time_ordering when reading sigmet file with extended headers
MAINT: refactor of _sigmet_noaa_hh module
ENH: radar.info prints airborne parameters
ENH: Default metadata for airborne parameters
MAINT: cleanup of coord_transform module
MAINT: clean up of radardisplay_airborne module
DOC: added airborne modules to developer reference manual