[ODE] Collision rule

[Erwin de Vries]

Quoted from the TODO list:

this collision rule:
    // no contacts if both geoms on the same body, and the body is not 0
    if (g1->body == g2->body && g1->body) return 0;
needs to be replaced. sometimes we want no collision when both bodies are 0,
but this wont work for geomgroup-to-environment.

----

'A' solution would be to check the body pointers in the lowest level
collision functions.

Erwin