Support building gexf.c into igraph

[ruebot]

Follow-on to #28;

The commented out code is to support building into Igraph. Igraph requires graph level attributes (eg. "totalNodes") to accept a PR, but the Gexf (even the 1.3) does not support graph-level attributes.

Since the schema for gexf is not rigidly enforced, it can be uncommented or removed without causing too much trouble.

The potential benefit to auk is if you would like to include some summary information in a sigma graph. For example, if you wanted to have some network summary information included in the visualisation.

In reality, I do not think it is high priority. I will fork the repo from HEAD and keep the leftover code there.