Graph::subgraphFromNodes with neighbors

[manpen]

This PR adds a new feature to Graph::subgraphFromNodes which originally created an induced subgraph on the set nodes of nodes provided. In some applications it however also makes sense to include their neighbours -- hence the new feature which can be enabled by setting the parameter includeOutNeighbors or includeInNeighbors. They default to false, so the standard behaviour does not change.

All edges in added must have at least one endpoint in nodes, the other one is either in nodes or in the set of neighbors included.

This PR also adds:

• CPP Unit tests from subgraphFromNodes
• Python Unit tests for subgraphFromNodes (with an invitation to add more Python test for Graph)
• Shebang line in all Python tests and exectuable flags, so we can easily execute those tests from the command line (if you do not like it in this PR, I can drop the commit and open another one).

[avdgrinten]

LGTM.

As a side note: subgraphFromNodes looks like a function that should really be in GraphTools.

[angriman]

I just have a minor comment, apart from that the PR looks good to me.

[manpen]

Thanks for the review. I added the comments (only change in the force-push).
@avdgrinten: I completely agree, and TBH, I first looked into GraphTools when I was searching this feature. There are additional methods such as merge (?), to(Unw|W)eighted, transpose, BFS that I do not really see as part of Graph. However, at the same time, these are typically methods used by user code and I do not really think it's worth breaking them.