Default units for 'UU' and 'VV' fields in `V2d`

Greetings, 

Just FYI:  While checking intermediate files using `rd_intermediate.exe`, I noticed the units for near the surface or 10 m u and v wind are `kg kg-1`. It traces back to `src/pywinter/winter.py`[ (lines 461-462):](https://github.com/dniloash/Pywinter/blob/175755263602f1f960b4fc89f1b4708440a4ad3a/src/pywinter/winter.py#L461). 

cheers,  