Add functionality to derive graph-specific scalar quantities

[RasmusOrsoe]

PyTorch Geometric have a pretty low focus on graph-specific quantities that can be derived off of graphs. There exists many additional quantities we could extract from events - both on a node-wise and graph-wise level.

Here is a list for inspiration: http://braph.org/braph-1-0/manual/graph-measures/

[RasmusOrsoe]

@BozianuLeon @kaareendrup @MortenHolmRep