Supporting the potential vorticity unit PVU

[marcowurth]

As I am not sure how would be the order in supporting this new unit I'll ask here.

I checked on the Default Pint units definition file and the PVU unit does not seems to exist yet in pint.

When then later supported it would be nice apart from being able to convert between the SI units and PVU to also have the calculation function metpy.calc.potential_vorticity_baroclinic() return PVU units. 1PVU equals 1.0 × 10-6 m2 s-1K kg-1 (see here)