code: def Lz(self,*args,**kwargs):
          
          tL= self.L(*args,use_physical=False)
          if len(tL.shape) == 2: # 3D orbit
              return tL[:,2]
          else: # 2D orbit
              return tL
docs: NAME:
             Lz
          PURPOSE:
             calculate the z component of the angular momentum
          INPUT:
             (none)
          OUTPUT:
             z-component of the angular momentum
          HISTORY:
             2018-08-29 - Written - Bovy (UofT)