平面8节点单元类，可以输出单元的刚度，并可通过形函数插值得到内部各点得位移、应力等。对各位以前编写的程序的一个补充和完善吧。(plane eight-unit category, the output unit stiffness, through the shape function can be interpolation points in the internal displacement and stress. Right before you and the procedure fo