GraphML support

[peterprokop]

Is it present/planned? If not I would be interested in adding it.

[davecom]

I think this would be great. I think we should do it along with support for Swift's new serialization protocols #22 . It could just be another encoder/decoder.

[johndpope]

maybe can cherry pick
https://github.com/Flaminis/Dissertation-Network-Analysis/blob/67a7712f433f98e0cc9a925f54452953dd332661/GraphViewer/GraphViewer/DisserGraphml.swift

[davecom]

Now that we have Codable support through the subclasses CodableUnweightedGraph and CodableWeightedGraph the next step would be to create a GraphML encoder/decoder.