Add demo_utils.

This commit splits the utils file into two different files:
utils and demo_utils, to overcome some opencv problems caused
by demo_utils includes.

Signed-off-by: Davide Sapienza <sapienza.dav@gmail.com>

demo/demo/demo3D.cpp

@@ -4,6 +4,7 @@
 //#include <unistd.h>
 #include <mutex>
 
+#include "demo_utils.h"
 #include "CenternetDetection3D.h"
 
 bool gRun;

demo/demo/demoTracker.cpp

@@ -4,6 +4,7 @@
 //#include <unistd.h>
 #include <mutex>
 
+#include "demo_utils.h"
 #include "CenterTrack.h"
 
 bool gRun;