#38 add support for a interest registry

[Raphael Ackermann]

To be able to filter out uninteresting nodes there has
to be a means of telling which nodes are interesting.

Interesting are all the nodes with distance < k from
the selected node.

after selection a traversal should happen starting at
the selected node, marking all the nodes interesting
that are < k distance away. They are marked interesting
by registering them.

Add support for such a registry

[Raphael Ackermann]

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reopened, as don't think it works just yet