In saturation_mixing_ratio, add Notes section and move citation for implementation to Notes. Also add an equation for the implementation following the citation.
In saturation_mixing_ratio, add Notes section and move citation for implementation to Notes. Also add an equation for the implementation following the citation.

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