Compute the rotation matrix R from the two vectors `a' (ax,ay,az) and `b' (bx,by,bz). `a' and `b' are the desired x and y axes of the rotated coordinate system. If necessary, `a' and `b' will be made unit length, and `b' will be projected so that it is perpendicular to `a'. The desired z axis is the cross product of `a' and `b'.