added msa to swiftgraph for pull request

[NickTheFreak97]

See #86

[NickTheFreak97]

Now .msa(_:) returns a set of edges, for a more uniform interface with respect to mst(_:).

EDIT: I know that 8k lines are a lot of work, but most of it are from test, run on a randomly generated completely connected graph with 2k vertices and about 7k edges and known MSA (about 7k lines), and an efficient complete implementation of LinkedList with O(1) insertion and deletions, given an iterator (about 700 lines). Let me know if I can help you somehow.

[NickTheFreak97]

Sorry for bothering you, any update or anything I could do for you to speed-up the review process?

[davecom]

Sorry for the late reply and thank you for the contribution. This seems like a fairly large amount of code surface (not in absolute terms but in terms of comparison to the existing library) for a fairly specific feature. And since I doubt there is a large amount of demand for it, it would seem like something that would be hard to find people to maintain over time for a medium size project like SwiftGraph. It will ultimately probably fall on me to update and since it's not a widely used algorithm I'm not sure it's something I would have a particular interest in having to maintain.

That's not to say it's not a good contribution, but just something that might not be general purpose enough to belong in the library. In short, I'm just hesitant to add something that won't get maintained and seems niche enough that it might belong in your own application-specific fork.