ENH: global read function

The pyart.io.read function determined the format of the file and then
calls the appropiate read_* function to create a Radar object.
ENH: added script to check CF/Radial compliance
io module fixes to better meet CF/Radial standard
updated check_cfradial.py script
STY: renamed PyGrid to Grid
Merge pull request #63 from jjhelmus/master

read function,  better CF/Radial support, script to check CF/Radial compliance