Report how far a change spreads: change radius and propagation cost #58

How far does a typical change spread? Report the change radius (how many modules a logical change touches: median, p90, share that stays in one module) for the repository and per module, and the propagation cost (average share of the code a change to one file drags along, from the co-change graph, MacCormack et al. 2006). A design that holds keeps most changes inside one area.