Handling partial ground truth

[ChristopherSchmidt89]

As some models are only partially validated from domain experts, we should allow for ground truth graphs, which present a subset of all available variables in a dataset. Marking those edges/nodes as "unknown" if there is neither a validated or invalidated edge set in the ground truth.

Possibly we should assume this as a default behavior and explicitly manage all edges either as "should be in the model", "should definitively not be in the model"; default "unknown".