plumb URDF/SDF audio_source into PhysicsServerCommandProcessor, allow to play sounds on collision !

See also https://youtu.be/eppOjTfx5Jg for a first test, and this modified URDF how to add sounds:
https://github.com/bulletphysics/bullet3/blob/master/data/plane_with_collision_audio.urdf
Add the --audio flag to enable sound in pybullet/Bullet-C-API

src/LinearMath/btHashMap.h

@@ -79,6 +79,11 @@ class btHashInt
 {
 	int	m_uid;
 public:
+
+	btHashInt()
+	{
+	}
+
 	btHashInt(int uid)	:m_uid(uid)
 	{
 	}