Create GraphML export

[pgleeson]

GraphML is used as an import format in applications like Gephi and Cytoscape. It would be useful to export networks (at multiple levels of detail) from NeuroMLlite to this format for use in these (hopefully high performance) graphing applications.

A new Writer class could be created for this, but the existing GraphViz based writer could also be reused with an option for output format, to keep the levels (nodes only; nodes + conns; conns + info, etc) used there as options for export, e.g.

python Example6.py -graph2c  # Export to GraphViz format, level 2 and generate png image with circo engine
python Example6.py -graphml2 # Export to GraphML, level 2

The GraphML could also eventually be used with http://js.cytoscape.org/ to make graphs to embed in Open Source Brain network visualisations...