Added simple GraphML sample file
Added simple GraphML sample file

The file was obtained from the `Prefuse` GitHub repository [1]. It seems to
correspond to the example file on the Gephi website [2].

[1]: https://github.com/prefuse/Prefuse/blob/master/data/graphml-sample.xml
[2]: https://gephi.org/users/supported-graph-formats/graphml-format/

Submanifold/Aleph,Submanifold/Aleph,Submanifold/Aleph