code: def _zforce(self,R,z,phi=0.,t=0.):
          
          return -z/(R**2.+z**2.)**(self.alpha/2.)
docs: NAME:
             _zforce
          PURPOSE:
             evaluate the vertical force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the vertical force
          HISTORY:
             2010-07-10 - Written - Bovy (NYU)