SITL: added SIM_TIME_JITTER parameter

for adding timing jitter in SITL

libraries/SITL/SITL.cpp

@@ -247,6 +247,8 @@ const AP_Param::GroupInfo SIM::var_info3[] = {
     AP_SUBGROUPINFO(baro[2], "BAR3_", 36, SIM, SIM::BaroParm),
 #endif
 
+    AP_GROUPINFO("TIME_JITTER",  37, SIM,  loop_time_jitter_us, 0),
+
     // user settable parameters for the 1st barometer
     // @Param: BARO_RND
     // @DisplayName: Baro Noise

libraries/SITL/SITL.h

@@ -230,6 +230,7 @@ public:
     AP_Int32 mag_devid[MAX_CONNECTED_MAGS]; // Mag devid
     AP_Float buoyancy; // submarine buoyancy in Newtons
     AP_Int16 loop_rate_hz;
+    AP_Int16 loop_time_jitter_us;
     AP_Int32 on_hardware_output_enable_mask;  // mask of output channels passed through to actual hardware
 
 #ifdef SFML_JOYSTICK