Training to form triplets

A next GNN just after the one that filter doublets will be implemented.

1. Filter edges with a given score_cut, favouring efficiency over purity
2. The remaining doublets (or edges) become the nodes of the GNN.
3. Edges between doublets (= nodes of our new GNN) are created between doublets that share a hit.

This procedure was described in one of the Exa.TrkX paper.