[bboxtree] Improve geometry collision and return intersection corners
The geoemtry collision noe return a vector of intersection corners. The collision algorithm with another budning box tree returns a vector of BoundingBoxTreeIntersection object. The object has a method first and second to get the intersecting entity indices as well as a method corners. From that you could build e.g. a intersection geometry. The intersections are not unique yet. There might be interesctions with the same corner points but different elements.