FUNCTION: thm_cal_rot(v1,v2) INPUT: v1: 3 component vector, v2: 3 component vector, PURPOSE: Returns a rotation matrix that rotates v1,v2 to the x-y plane v1 is rotated to the x-axis and v2 into the x-y plane NOTES: Function normally called by "add_df2d.pro" to rotate velocities into the plane of Vsw and B CREATED BY: J.McFadden LAST MODIFICATION: 95-9-13 J.McFadden