Fixed BoxBox Intersection Algorithm

The boxBox intersection algorithm had bugs in it. After comparing it to the bullet implementation, everything is now fixed. When there are multiple points of collision between two boxes, the deepest point of collision is returned.
The first 6 cases had (s2 > 0). To keep things consistent I changed the remaining 9 cases to also use (s2 > 0). In the bullet implementation s2 is compared to the shape_expansion variable, but FCL doesn't seem to have anything like it.