code: def vphi(self,*args,**kwargs):
          
          thiso= self(*args,**kwargs)
          if not len(thiso.shape) == 2: thiso= thiso.reshape((thiso.shape[0],1))
          return thiso[2,:]/thiso[0,:]
docs: NAME:
             vphi
          PURPOSE:
             return angular velocity
          INPUT:
             t - (optional) time at which to get the angular velocity
             vo= (Object-wide default) physical scale for velocities to use to convert
             use_physical= use to override Object-wide default for using a physical scale for output
          OUTPUT:
             vphi(t)
          HISTORY:
             2010-09-21 - Written - Bovy (NYU)