[CF-metadata] Standard name for radial velocity

[Jonathan Gregory]

Dear Ag

> After much discussion the best agreement we can come up with for a standard
> name for the instrument-measured quantity of radial velocity is:
> 
>   'radial_velocity_of_scatterers_away_from_instrument'  (m s^-1)

Thanks for coordinating that discussion, which was useful. I certainly think
we should add this to the standard name table.

Cheers

Jonathan