code: def turn_physical_on(self,ro=None,vo=None):
          
          self._roSet= True
          self._voSet= True
          if not ro is None:
              self._ro= ro
          if not vo is None:
              self._vo= vo
          return None
docs: NAME:
             turn_physical_on
          PURPOSE:
             turn on automatic returning of outputs in physical units
          INPUT:
             ro= reference distance (kpc)
             vo= reference velocity (km/s)
          OUTPUT:
             (none)
          HISTORY:
             2016-01-19 - Written - Bovy (UofT)