Copter: bug fix to look_ahead_yaw

Fix received from David Dewey
Was incorrectly using the direction instead of heading when deciding
whether to point forward or not.  Reduced YAW_LOOK_AHEAD_MIN_SPEED to
1m/s because bug meant it was nearly always pointing forward and 10m/s
(the old value) seemed too high

ArduCopter/Attitude.pde

@@ -762,7 +762,7 @@ static void get_look_at_yaw()
 static void get_look_ahead_yaw(int16_t pilot_yaw)
 {
     // Commanded Yaw to automatically look ahead.
-    if (g_gps->fix && g_gps->ground_course_cd > YAW_LOOK_AHEAD_MIN_SPEED) {
+    if (g_gps->fix && g_gps->ground_speed_cm > YAW_LOOK_AHEAD_MIN_SPEED) {
         nav_yaw = get_yaw_slew(nav_yaw, g_gps->ground_course_cd, AUTO_YAW_SLEW_RATE);
         get_stabilize_yaw(wrap_360_cd(nav_yaw + pilot_yaw));   // Allow pilot to "skid" around corners up to 45 degrees
     }else{

ArduCopter/config.h

@@ -781,7 +781,7 @@
 #endif
 
 #ifndef YAW_LOOK_AHEAD_MIN_SPEED
- # define YAW_LOOK_AHEAD_MIN_SPEED  1000             // minimum ground speed in cm/s required before copter is aimed at ground course
+ # define YAW_LOOK_AHEAD_MIN_SPEED  100             // minimum ground speed in cm/s required before copter is aimed at ground course
 #endif