code: def z(self,*args,**kwargs):
          
          if len(self.vxvv) < 5:
              raise AttributeError("linear and planar orbits do not have z()")
          thiso= self(*args,**kwargs)
          onet= (len(thiso.shape) == 1)
          if onet: return thiso[3]
          else: return thiso[3,:]
docs: NAME:
             z
          PURPOSE:
             return vertical height
          INPUT:
             t - (optional) time at which to get the vertical height
             ro= (Object-wide default) physical scale for distances to use to convert
             use_physical= use to override Object-wide default for using a physical scale for output
          OUTPUT:
             z(t)
          HISTORY:
             2010-09-21 - Written - Bovy (NYU)