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