MNT: Replace assert with log message in NEXRAD
MNT: Replace assert with log message in NEXRAD

We're just checking a size here, so be less aggressive and warn rather
than error out. It's completely possible to be off a little in size and
still parse through the file fine.

ahaberlie/MetPy,dopplershift/MetPy,dopplershift/MetPy,jrleeman/MetPy,Unidata/MetPy,jrleeman/MetPy,ShawnMurd/MetPy,ahaberlie/MetPy,Unidata/MetPy