MNT: Replace **2 with a * a.
MNT: Replace **2 with a * a.

Calling out to pow is unnecessary, and can trigger some more challenging
unit behavior.

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