New gml reader and writer

This idea is born from issue #972 and the discussion followed suit. This push aims to replace the old GML file format parser and generator with one stricter, faster and less buggy.

- it doesn't have external dependencies
- supports attribute lists – unlike what the `write_gml` documentation says, GML supports list attributes as long as they have at least two elements.
- ensures node ids are integers, if not sets the label as the old id value and assigns an increasing number as id.
- it's more tested.

I'm naturally opened to discussion and modifications.

Luca :)

The result of `G.to_directed()` can never be accessed outside this function.