TST: Add tests for humidity functions
TST: Add tests for humidity functions

We were not testing that these functions accepted plain numbers, and
there are special code paths for this in the implementations.

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