Colliding geometry is only in CollidingObject.

To do this, we also had to :
- have PbdObject inherit CollidingObject (in the future
there should not be a distinct object class just for Pbd :
there should just be dynamical objects that can use
different models, like Pbd or FEM)
- remove useless checks when creating interactions: even
two colliding objects (not dynamic, aka static) could
be interacting if they are both controlled by devices