Add GraphmlOutput

[ccossou]

I've created a new Output graphmlOutput, to create a .graphml file that you can open in Yed).

I don't know if you're interested to add this to your package. Just in case I made a pull request. You'll find below the limitations and how to use.

The layout is not done, you need to do the layout in a GUI, in 3 simple steps:

• Open the .graphml file you produced in Yed
• do Tools>Fit Node to Label
• In the Layout menu, use one of the layouts available to properly display the nodes that are currently overlapping one another. A good start is to use either "One Click Layout" or "Hierarchical".

The idea is to have an easily editable graph.

important informations:

• I added a dependency with Pyed, hope I did it right, because I wasn't sure where you stored the 'normal' dependencies
• Check the colors and change the values if needed
• unit tests? I don't know how to test the Output and open for ideas
• This Output is probably not compatible with asynchroneous. I don't know how to safeguard against this (you can't make the edges or the groups before you've processed all the nodes
• I rewrote the node_label function because "\r" prevented new lines to appear in my graph labels. I then stripped number of calls and time from the analysis because I prefered that way, but this can easily be added.