Quat2Euler

代码说明：

  将Quaternion四元素坐标系转换为一般的欧拉角坐标系，避免采用反三角函数导致的值域限制问题。参数xi为搜索的精度，对于每个欧拉角可以分别设定，xi越小，搜索越精确。(Quaternion quaternion coordinate system will be converted to general Euler angle coordinate system, avoiding the use of inverse trigonometric functions due to range limitations. The accuracy of the search parameter xi, for each Euler angle can be set separately, xi smaller, more accurate search.)