attribute a newly infected node to the source node

[yunshiuan]

Is your feature request related to a problem? Please describe.
I wonder if there's a way to attribute a newly infected node to the source node. That is, if a node gets infected, can I know which node infects it? This is very important when doing influence maximization or outbreak control on a cascading model.

Describe the solution you'd like
The model.iteration() function should return the source of each newly affected node. I believe this information should have been computed within the function scope, but isn't returned by the function.

Thank you!