SITL: added SIM_TERRAIN parameter

to choose either flat earth or AP_Terrain for sonar and optical flow
AGL emulation

libraries/SITL/SITL.cpp

@@ -62,6 +62,7 @@ const AP_Param::GroupInfo SITL::var_info[] PROGMEM = {
     AP_GROUPINFO("BARO_GLITCH",   31, SITL,  baro_glitch, 0),
     AP_GROUPINFO("SONAR_SCALE",   32, SITL,  sonar_scale, 12.1212f),
     AP_GROUPINFO("FLOW_ENABLE",   33, SITL,  flow_enable, 0),
+    AP_GROUPINFO("TERRAIN",       34, SITL,  terrain_enable, 1),
     AP_GROUPEND
 };
 

libraries/SITL/SITL.h

@@ -82,6 +82,7 @@ public:
 	AP_Int8  baro_disable; // disable simulated barometer
     AP_Int8  float_exception; // enable floating point exception checks
 	AP_Int8  flow_enable; // enable simulated optflow
+	AP_Int8  terrain_enable; // enable using terrain for height
 
     // wind control
     AP_Float wind_speed;