code: def _zforce(self, R, z, phi=0., t=0.):
          
          return -nu.sign(z) * self._denom(R,z)**-1.5 * (self._a + nu.fabs(z))
docs: NAME:
             _zforce
          PURPOSE:
             evaluate the vertical force  for this potential
          INPUT:
             R - Cylindrical Galactocentric radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the vertical force = -dphi/dz
          HISTORY:
             2016-05-09 - Written - Aladdin