WIP: refactor of read_nexrad_archive - 1
WIP: refactor of read_nexrad_archive - 2
WIP: refactor of read_nexrad_archive - 3
ENH: read_nexrad_archive returns a single radar object

* Gates/Moments not collected are masked in the returned object
WIP: read_nexrad_cdm refactor
ENH: read_nexrad_cdm returns a single radar object

* Gates/Moments not collected are masked in the returned object
DOC: nexrad documentation
Merge branch 'nexrad_level2'