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.
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
BUG: azimuth and elevation returned as degrees for airborne sigmet radar
Merge branch 'jjhelmus-airborne' into airborne

Merging with jjhelmus update of airborne part of code.