[BUG] Some nodes may be parents of themselves

[levnikmyskin]

As per the title, when not following the "preferred" parents path, some nodes may end up having themselves as their own ancestor. This will lead to an infinite loop in the hierarchy construction (and will bring to a segmentation fault in the Python module).

For instance, subject 300036794 has subject 300212545 among its additional parents. Subject 300212545 has then subject 300036794 among its additional ancestors, creating an infinite loop.

This doesn't seem to happen if one follows the "preferred" hierarchy only and one simple but bad solution might be to reconstruct this type of hierarchy only. Something else should be thought out to deal with such problems