simulation code - step3, decentralized SGD as a reference implementation, asynchronous / time-varying graphs

[martinjaggi]

depends on #1 . related to #2 also.

modify the reference code (simulated decentralized) for a given communication graph, on a standard/toy dataset.

incorporate a basic asynchronous model, i.e. allowing node and edge failures in SGD, or in other words a few variants time-varying graphs. this can also be used to simulate some realistic notions of fault tolerance

this will be used later to compare the p2p version to it, and to test different algorithm variants before implementing them in the real p2p framework