Added BellmanFord algorithm

[spirosmaggioros]

Ok so here is what i think, you can implement this algorithm inside graph.h header file. Then you can rename your file in the examples folder as "negative_cycles.cc". So in short, just carry the implementation to graph.h and create the graph with the graph<T> class and make an example with negative cycles. Thank you for your interest and your time, we'll be here for further help if you need so 💯

[arthiondaena]

There was a "std::chrono has not been declared" error so i have include thread library