Fixed 'sinks' method for undirected graph

[lynxaegon]

Fixes #101

[assafsun]

interesting issue :)
I'm trying to think is there is a meaning of sinks and sources in undirected graph as they are equal right?

[dforsber]

interesting issue :)
I'm trying to think is there is a meaning of sinks and sources in undirected graph as they are equal right?

What should sources() and sinks() return for a list of nodes without edges? Both return all the nodes?

[lynxaegon]

interesting issue :)
I'm trying to think is there is a meaning of sinks and sources in undirected graph as they are equal right?

What should sources() and sinks() return for a list of nodes without edges? Both return all the nodes?

@dforsber from the API Reference:

# graph.sources()
Returns those nodes in the graph that have no in-edges. Takes O(|V|) time.

# graph.sinks()
Returns those nodes in the graph that have no out-edges. Takes O(|V|) time.

@assafsun is right, in an undirected graph, in-edges and out-edges are the same.

I was using graphlib for a mesh network graph (undirected ofc) and i was using sinks to determine that a node in the network wasn't connected to anything anymore (on disconnects). The sinks term was just better for me.
Looking back, i should probably update the PR to make the sources() and sinks() do the same thing for an undirected graph

[rustedgrail]

I updated the PR, but the new test doesn't pass. Can you take a look?

#220