ENH: Reading of NEXRAD Level II data

Support for reading NEXRAD Level 2 data file from: NCDC and the THREDDS server (both online and static).

New functions documented at

http://arm-doe.github.io/pyart/dev/user_reference/generated/pyart.io.read_nexrad_archive.html#pyart.io.read_nexrad_archive

http://arm-doe.github.io/pyart/dev/user_reference/generated/pyart.io.read_nexrad_cdm.html#pyart.io.read_nexrad_cdm