algo::bellman_ford should return the source and target indices when returning the NegativeCycle error.

[mitchmindtree]

This can be useful when using bellman_ford to find negative cycles (e.g. in exchange arbitrage).

[bluss]

😄 That's a fun idea, but I will actually close this. We don't need all loose ideas as issues. If this is an actual use case that you are pursuing, make sure to say so in the issue (and it will not be closed).

As always, I hope that algorithms can be built upon petgraph and just not in petgraph.