Use sdf files instead of urdf files

Fix

Tools/gazebo_sitl_multiple_run.sh

@@ -66,9 +66,10 @@ while [ $n -lt $num_vehicles ]; do
 		rotors_description_dir:=${src_path}/Tools/sitl_gazebo/models/rotors_description mavlink_udp_port:=$(($mavlink_udp_port+$n)) \
 		mavlink_tcp_port:=$(($mavlink_tcp_port+$n))  -o /tmp/${PX4_SIM_MODEL}_${n}.urdf
 
+	gz sdf -p  /tmp/${PX4_SIM_MODEL}_${n}.urdf > /tmp/${PX4_SIM_MODEL}_${n}.sdf
 	echo "Spawning ${PX4_SIM_MODEL}_${n}"
 
-	gz model --spawn-file=/tmp/${PX4_SIM_MODEL}_${n}.urdf --model-name=${PX4_SIM_MODEL}_${n} -x 0.0 -y ${n} -z 0.0
+	gz model --spawn-file=/tmp/${PX4_SIM_MODEL}_${n}.sdf --model-name=${PX4_SIM_MODEL}_${n} -x 0.0 -y ${n} -z 0.0
 
 	popd &>/dev/null