BUG: Fix unit-handling in heat_index (Fixes #1867)
BUG: Fix unit-handling in heat_index (Fixes #1867)

Was changed in c7b0b9b to remove multiplication by units and slightly
change the operations used--this runs afoul of upstream changes in Pint
where a Quantity is passed as the data in a constructor. Instead, change
the units on a constant to produce the required dimensionality properly.

Unidata/MetPy,dopplershift/MetPy,dopplershift/MetPy,Unidata/MetPy