Add NEXRAD Support

[mgrover1]

Description

Add a backend for NEXRAD level 2 and level 3 files

What We Want to Do

NEXRAD data is heavily used within the radar community, and it would be helpful to have a reader for this data in xradar.

Py-ART has readers for this, including the

• [x] #160
• [ ] NEXRAD Level 3 Reader

@jthielen mentioned he would be interested in helping with this (see #27)... others can help too!