Add GraphML UDF

[greebie]

Currently we have two network outputs -- GDF & GEXF.

GEXF is a standard needed for Sigma.js and is fairly standard across applications. The other standard is GraphML.

GDF was popular primarily due to a service that created ego networks from Facebook. This service is now defunct and the GDF format is not widely used.

I suggest 1) Add a GraphML UDF
and 2) Deprecating the GDF output, recommending either GEXF or GraphML which are used much more widely and have more features for creating networks (including support for nested and dynamic networks if we want to use those down the line).

[ruebot]

Resolved with: 116b053