Evaluation of the obligation of the structure node

It has to be sure that all parents are visible and accessible as well.
This wasn't achieved sometimes in the current evaluation logic.
Additionally I simplified the learning path access algorithm thank's
the usage of the stored assessment data.

... instead of the completion and status of the current run.

The NodeEvaluation makes it difficult to use a course node in the
learning path access concept. So it is refactored to a minimum extent,
so that it can covering the previous functionality.
As a replacement a new security callback and a new tree node type is
introduced.