unit of vorticity in code table 3.15

[manfredsc]

Branch

https://github.com/wmo-im/GRIB2/tree/iss116

In code table 3.15, the unit of potential vorticity is depicted as "K m-2 kg-1 s-1". I think this is wrong, it should be "K*m^2/kg/s", or in your notation, "K m2 kg-1 s-1".

[amilan17]

@jitsukoh confirmed

[amilan17]

@jitsukoh branch is ready.

[jitsukoh]

@amilan17 I confirm that the branch is updated and the issue is moved to the validated status.