Graphml Runner

[pkel]

This PR adds the graphml_runner executable which takes network, protocol and simulator arguments as graphml (xml) input, runs the simulation, and writes the result to stdout as graphml.

This is useful for coordinating experiments from python and R.

The igraph runner replaces the old igraph experiment.

• [x] Fix old igraph experiment as reference
• [x] Code igraph_runner.exe
• [x] Rename igraph runner to graphml runner
• [x] Demonstrate how to use the runner from python
• [x] Demonstrate how to use the runner from R
• [ ] Reproduce old igraph experiment with new runner
• [ ] Delete old igraph experiment

[pkel]

Spent quite some time on rebasing this today. I'll merge this now. The two open points can be dealt with later.