code: def _2ndderiv_xyz(self,x,y,z,i,j):
          
          return -np.pi*self._rhoc_M*self.a**3*self._b*self._c *\
              _2ndDerivInt(x,y,z,self._a2,self._b2*self._a2,self._c2*self._a2,self.n,i,j)
docs: General 2nd derivative of the potential as a function of (x,y,z)
          in the aligned coordinate frame, d^2\Phi/dx_i/dx_j