Ball collision with robot chassis is not being handled

[FelipeMartins96]

Describe the bug
Ball collision with robot chassis is not being handled

To Reproduce
Steps to reproduce the behavior:

1. Start grSim
2. Move ball in robot direction, between wheels
3. Observe the ball getting inside the robot

Expected behavior
Ball should not get inside the robot

Screenshots

Additional context
The code creating a surface between the ball and robot chassis is currently commented on the code.

grSim/src/sslworld.cpp

Line 284 in 8d3b8c7

//p->createSurface(robots[k]->chassis,ball);

I tested uncommenting this line, the ball stopped getting inside the robot, but it stopped contacting properly with the kicker.

[FelipeMartins96]

I've got some ideas to test, but I was wondering what this callback does

grSim/src/sslworld.cpp

Lines 116 to 133 in 8d3b8c7

	bool ballCallBack(dGeomID o1,dGeomID o2,PSurface* s, int /*robots_count*/)
	{
	if (_w->ball->tag!=-1) //spinner adjusting
	{
	dReal x,y,z;
	_w->robots[_w->ball->tag]->chassis->getBodyDirection(x,y,z);
	s->fdir1[0] = x;
	s->fdir1[1] = y;
	s->fdir1[2] = 0;
	s->fdir1[3] = 0;
	s->usefdir1 = true;
	s->surface.mode = dContactMu2 | dContactFDir1 | dContactSoftCFM;
	s->surface.mu = _w->cfg->BallFriction();
	s->surface.mu2 = 0.5;
	s->surface.soft_cfm = 0.002;
	}
	return true;
	}

It seems that the ball->tag value is only set to -1 on the simulation step, and never changed to another value, so as far as I can see this callback is never running