SITL: added a SIM_GPS_HZ option

this allows simulation of slow GPS updates (for example, the 1Hz GPS
problem)

libraries/AP_HAL_AVR_SITL/sitl_gps.cpp

@@ -531,7 +531,7 @@ void SITL_State::_update_gps(double latitude, double longitude, float altitude,
     Vector3f glitch_offsets = _sitl->gps_glitch;
 
 	// 5Hz, to match the real config in APM
-	if (hal.scheduler->millis() - gps_state.last_update < 200) {
+	if (hal.scheduler->millis() - gps_state.last_update < 1000/_sitl->gps_hertz) {
 		return;
 	}
 

libraries/SITL/SITL.cpp

@@ -43,6 +43,7 @@ const AP_Param::GroupInfo SITL::var_info[] PROGMEM = {
     AP_GROUPINFO("MAG_ERROR",     15, SITL,  mag_error,  0),
     AP_GROUPINFO("SERVO_RATE",    16, SITL,  servo_rate,  0),
     AP_GROUPINFO("GPS_GLITCH",    17, SITL,  gps_glitch,  0),
+    AP_GROUPINFO("GPS_HZ",        18, SITL,  gps_hertz,  5),
     AP_GROUPEND
 };
 

libraries/SITL/SITL.h

@@ -62,6 +62,7 @@ public:
     AP_Int8  gps_type;    // see enum GPSType
     AP_Float gps_byteloss;// byte loss as a percent
     AP_Int8  gps_numsats; // number of visible satellites
+    AP_Int8  gps_hertz;   // GPS update rate in Hz
     AP_Vector3f  gps_glitch;  // glitch offsets in lat, lon and altitude
 
     // wind control