Option to calculate MSD for each node

[MSD](https://en.wikipedia.org/wiki/Mean_Shortest_Distance#Mean_shortest_distance) seems like a common convention for determining how 'trusted' a key is. It might be helpful to specify add an additional flag that calculates the MSD for each node and assigns it as an attribute. There's probably something that fits our needs [here](http://networkx.github.io/documentation/latest/reference/algorithms.shortest_paths.html)