Prediction graph

A predication graph is a graph where the edges contain the values of the trust predicted by a trust metric through the leave-one-out technique. It contains a trust network with the original nodes and edges. On an edge (a, b) there is both the original trust value ['orig'] from a to b but also the predicted trust value ['pred'] predicted by the trust metric for (a, b), by leaving out edge (a, b). If a prediction was not possible, the predicted trust is None. There are two types of Prediction Graph, the first is the PredGraph class, used for generic Network and for the Advogato_like_network, the second is the WikiPredGraph class, used for the WikiNetwork