NoBackwardLink constraint not working as expected & modification

[Miguel Ángel Artaso Landa]

The NoBackwardLink constraint is not working as expected and this should be checked. It would be necessary also to modify its behavior so it allows to create links from the nodes located in the slice 0 heading to atemporal nodes.

[Miguel Ángel Artaso Landa]

The necessary code has been added to the constraint. Commit hash b5c7011b41f71e3dfc1a7cdec0ba1909735f7efc