GDF file format support in readwrite

[stijn-uva]

This PR adds support for the GDF network file format via the functions read_gdf, parse_gdf, write_gdf and dump_gdf.

An old issue #455 exists for this but was closed at the time because the two main applications known to use this format - Gephi and GUESS - also support other formats that NetworkX can read/write. However, some of the research software we use (e.g. 4CAT) only supports GDF, and support for it in NetworkX would make it easier for people to use the output of that software, and would allow us to integrate NetworkX's functionality in our tools.

This is my first PR for NetworkX so please tell me if extra work would be needed before it can be merged!

[rossbar]

From a quick search on the internet I'm inclined to agree with the conclusion in #455 - it might be more appropriate to add support for more commonly used network data formats to 4CAT than to add support for yet another not-particularly-common (AFAICT) text-based format to NetworkX. For example, it seems that TCAT, which appears to have inspired 4CAT, supports I/O in GEXF format.

[MridulS]

I agree with @rossbar, thanks for the PR!! but I think we shouldn't be adding another text based graph format which seemingly (from a cursory internet search) doesn't seem to be supported in a lot of places. It heavily increases our maintenance burden.

And it looks like 4CAT has moved to GEXF digitalmethodsinitiative/4cat@176b15a ?

Anyway I'll go ahead and close this PR, thanks again for your contribution!