SITL: added SIM_RATE_HZ

this allows the physics step size to be changed while flying the
internal models, which allows for lower CPU usage

libraries/SITL/SIM_Aircraft.cpp

@@ -598,6 +598,9 @@ void Aircraft::update_dynamics(const Vector3f &rot_accel)
         }
         }
     }
+
+    // allow for changes in physics step
+    adjust_frame_time(constrain_float(sitl->loop_rate_hz, rate_hz-1, rate_hz+1));
 }
 
 /*

libraries/SITL/SITL.cpp

@@ -257,6 +257,8 @@ const AP_Param::GroupInfo SITL::var_info3[] = {
     // vicon velocity glitch in NED frame
     AP_GROUPINFO("VICON_VGLI",    21, SITL,  vicon_vel_glitch, 0),
 
+    AP_GROUPINFO("RATE_HZ",  22, SITL,  loop_rate_hz, 1200),
+
     AP_GROUPEND
 
 };

libraries/SITL/SITL.h

@@ -194,6 +194,7 @@ public:
     AP_Float mag_scaling; // scaling factor on first compasses
     AP_Int32 mag_devid[MAX_CONNECTED_MAGS]; // Mag devid
     AP_Float buoyancy; // submarine buoyancy in Newtons
+    AP_Int16 loop_rate_hz;
 
     // EFI type
     enum EFIType {