code: def rl(self,lz):
          
          if _APY_LOADED and isinstance(lz,units.Quantity):
              lz= lz.to(units.km/units.s*units.kpc).value/self._vo/self._ro
          return rl(self,lz,use_physical=False)
docs: NAME:
          
              rl
          
          PURPOSE:
          
              calculate the radius of a circular orbit of Lz
          
          INPUT:
          
          
              lz - Angular momentum (can be Quantity)
          
          OUTPUT:
          
              radius
          
          HISTORY:
          
              2012-07-30 - Written - Bovy (IAS@MPIA)
          
          NOTE:
          
              seems to take about ~0.5 ms for a Miyamoto-Nagai potential; 
              ~0.75 ms for a MWPotential