Undirected Graph Support in Cassovary

[plofgren]

Related to #140
The idea of this wrapper is that each graph implementation shouldn't need custom code for StoredGraphDir.Mutual. We can write a single wrapper which works with any graph implementation.
Here is an initial undirected graph class. If this looks good, I can add a test and support for mutable undirected graphs to this pull request.
@pankajgupta

[pankajgupta]

pls add the standard copyright header you can get from other files (except change 2014 to 2015)

[plofgren]

Done

[pankajgupta]

@szymonm : I was also just thinking that, and agree that parametrizing Graph with NodeType will be better. Let's do that in a separate PR. See my comment above, we can do getNodeById to return UndirectedNode in UndirectedGraph regardless.

[szymonm]

On the other hand, I used: https://github.com/scala-graph/scala-graph.

They use:

trait GraphLike[
  N,
  E[X]  <: EdgeLikeIn[X], 
  +This[X, Y[X]<:EdgeLikeIn[X]] <: GraphLike[X,Y,This] with AnySet[Param[X,Y]] with Graph[X,Y]
]

and IMO this type magic makes the library very hard to use...

So let's be carefull about that.

[pankajgupta]

:) Agreed that there's a balance. Parametrizing by NodeType is good I think.

[plofgren]

(Sorry for the delay). I changed UndirectedGraph to a trait, made UndirectedNode a sub-trait of UnidirectedNode, and made UndirectedGraph's methods return UndirectedNode. I think that addresses all of your comments. I also just rebased from master.

[pankajgupta]

Ouch, somehow I missed your updated commits, Peter!
Also, looks your rebase has gone awry as the diff in this PR is showing other changes unrelated to your change. May I ask you to build a new patch or create a clean one on top of current master. Thanks.

[pankajgupta]

Ping @plofgren

[plofgren]

To be honest, I don't think I'll get back to this until at least after I finish my PhD in a couple months. It doesn't really add functionality, so I propose we just drop it and optionally change the documentation as suggested in #140.

[pankajgupta]

Ah, you have done a lot of work. Let's leave this open for now and hope you come back to it when you get time. Or someone else might pick it up too.

[plofgren]

Sounds good.

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