supporting attributes for GraphML formats

[filchristou]

I would like to be able to read a metagraph from a graphML format. This means reading in all attributes and properties that a graphML file may contain, and storing them in a metagraph. I 've been playing around for the last hours with the package and I got it going.

Obviously the packet dependencies will grow (MetaGraphs)

The implementation is done using XPath and DOM and not the EzXML.StreamReader for the reason that there needs to be some memory on the parser in order to remember whose are the properties read. I don't argue that it is impossible to do with EzXML.StreamReader and I also understand that reading all structure in RAM is more resource-hungry. I just implemented the obvious solution

Is it fine if I do a PR ?