code: def set_vo(vo):
      
      if _APY_LOADED and isinstance(vo,units.Quantity):
          vo= vo.to(units.km/units.s).value
      __config__.set('normalization','vo',str(vo))
docs: NAME:
         set_vo
      PURPOSE:
         set the global configuration value of vo (velocity scale)
      INPUT:
         vo - scale in km/s or astropy Quantity
      OUTPUT:
         (none)
      HISTORY:
         2016-01-05 - Written - Bovy (UofT)