code: def _zforce_xyz(self,x,y,z):
          
          return -2.*np.pi*self._rhoc_M * self.a**3*self._b*self._c * \
              _forceInt(x, y, z, self._a2, self._b2*self._a2, self._c2*self._a2, self.n, 2)
docs: Evaluation of the z force as a function of (x,y,z) in the aligned
          coordinate frame