Added a function to calculate pressure-weighted mean wind in a layer.
Added a function to calculate pressure-weighted mean wind in a layer.

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