电流环磁场

代码说明：

  把区域划分成矩形网格，找到每个网格的中心点，把每个网格当做一个圆环计算每个圆环的磁场，然后调用linprog函数就求出场分布了。不要求你把线圈矩形化，也不要求屏蔽场。(The region is divided into rectangular grids and the center points of each grid are found. Each grid is used as a circle to calculate the magnetic field of each ring, and then the field distribution is called by the linprog function. It does not require you to rectangle the coil, nor does it require shielding.)