Add the ability to have trailing empty members on NamedStruct.
Add the ability to have trailing empty members on NamedStruct.

This allows them to be filled in later.

dopplershift/MetPy,ahaberlie/MetPy,jrleeman/MetPy,ahill818/MetPy,Unidata/MetPy,jrleeman/MetPy,ShawnMurd/MetPy,dopplershift/MetPy,Unidata/MetPy,ahaberlie/MetPy,deeplycloudy/MetPy