code: def _zforce(self,R,z,phi=0.,t=0.):
          
          m= 4.*R*self.a/((R+self.a)**2+z**2)
          return -4.*z*self.a/(1.-m)*((R+self.a)**2+z**2)**-1.5*special.ellipe(m)
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:
             2018-08-04 - Written - Bovy (UofT)