Move UML to its own repo

[clemens-tolboom]

As UML is not related to Graph data we should move it out of Graph project

[clemens-tolboom]

It's Graphviz Record structure should be weakend as that is useful of it's own like for a database relation.

[clue]

I've raised a couple of related questions in ticket #25 which should be addressed there first.

Otherwise, I'd be happy to move the UML part to a new repo (which btw was planned since at least 2012-06-07: https://github.com/Flos/graph-php/issues/9)