SITL: added SIM_LOOP_DELAY parameter

this allows us to test high CPU load conditions by adding a delay to
each loop

libraries/SITL/SITL.cpp

@@ -186,6 +186,9 @@ const AP_Param::GroupInfo SITL::var_info2[] = {
     AP_GROUPINFO("OPOS_ALT",    53, SITL,  opos.alt, 584.0f),
     AP_GROUPINFO("OPOS_HDG",    54, SITL,  opos.hdg, 353.0f),
 
+    // extra delay per main loop
+    AP_GROUPINFO("LOOP_DELAY",  55, SITL,  loop_delay, 0),
+
     AP_GROUPEND
 
 };

libraries/SITL/SITL.h

@@ -176,6 +176,7 @@ public:
     AP_Float flow_noise; // optical flow measurement noise (rad/sec)
     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
 
     // wind control
     enum WindType {