Exports OverflowDB Graph to GraphML

Note: GraphML doesn't natively support list property types, so we fake it by encoding it as a ; delimited string. If you import this into a different database, you'll need to parse that separately. In comparison, Tinkerpop just bails out if you try to export a list property to graphml.

https://en.wikipedia.org/wiki/GraphML http://graphml.graphdrawing.org/primer/graphml-primer.html