SITL: added SIM_MAG_SCALING for testing scale factor errors

# Conflicts:
#	libraries/SITL/SITL.cpp

libraries/SITL/SITL.cpp

@@ -189,6 +189,8 @@ const AP_Param::GroupInfo SITL::var_info2[] = {
     // extra delay per main loop
     AP_GROUPINFO("LOOP_DELAY",  55, SITL,  loop_delay, 0),
 
+    AP_GROUPINFO("MAG_SCALING",    60, SITL,  mag_scaling, 1),
+
     AP_GROUPEND
 
 };

libraries/SITL/SITL.h

@@ -177,6 +177,7 @@ public:
     AP_Int8  baro_count; // number of simulated baros to create
     AP_Int8 gps_hdg_enabled; // enable the output of a NMEA heading HDT sentence
     AP_Int32 loop_delay; // extra delay to add to every loop
+    AP_Float mag_scaling; // scaling factor on first compasses
 
     // wind control
     enum WindType {