说明：  计算两组向量之间的旋转矩阵。输入参数为在两个不同坐标系中的同名向量（模为1），返回参数是一个3X3的旋转矩阵。
(The function of RM realize the rotation between two sets of vectors, whose magnitude are 1, defined in two different refrence frames. Input parameters are two sets of vectors r and b, return value is a rotaton matrix.)