利用J-A模型求解磁滞回线时，使用龙格库塔四阶方法求解微分方程的代码，并可以将求解结果直接绘制成磁滞回线图像-When the J-A model is used to solve the hysteresis loop, the Runge-Kutta fourth-order method is used to solve the differential equation code, and the solution result can be directly drawn as the hysteresis loop image