Support limiting collisions by group for softbody and particles

This feature is extremely useful for layering multiple cloth objects,
and there is no reason there shouldn't be the same kind of feature for softbody.

source/blender/blenkernel/intern/particle_system.c

@@ -3493,7 +3493,7 @@ static void dynamics_step(ParticleSimulationData *sim, float cfra)
 	psys_update_effectors(sim);
 
 	if (part->type != PART_HAIR)
-		sim->colliders = get_collider_cache(sim->scene, sim->ob, NULL);
+		sim->colliders = get_collider_cache(sim->scene, sim->ob, part->collision_group);
 
 	/* initialize physics type specific stuff */
 	switch (part->phystype) {