added a multiplication of "*trans_dir.z.signum()" for the z-direction of the...
requested to merge 302-inverted-reference-lens-reflects-instead-of-transmitting-in-martinez-opm into main
added a multiplication of "*trans_dir.z.signum()" for the z-direction of the normal vector of a plane intersection. otherwise, backwards refraction (transmission) did not work out correctly
Closes #302 (closed)