Define up-ward direction for ray_to_isometry on every ray-surface-intersection

There seems to be a problem when the ray travels exactly in y direction, as the isometry is defined with the "face_towards" function which needs to define an up-direction. This up-direction was set to y() per default. ideally, the initial up-direction is set to y() and then transformed with every positioning step of the nodes. For this, one may project the normal vector of the intersection onto the plane which is spanned by the new propagation direction of the reflected/refracted beam