Add fake root to RST-dep tree

It is common practice in dependency parsing to add an artificial root node.
This root node governs the (actual) root(s) of the dependency structure.
It is usually placed at the beginning of the input sequence.

This artificial root node cannot have incoming dependencies, which should solve root-related downstream issues, e.g. in attelo's MST decoder.

Another thing that puzzles me about this: if we always have a meaningful notion of root node in our dependency trees, why do we need to introduce a fake root node pointing just to that one node?  It's a bit more evident in the STAC case because you have a graph with multiple entry points, and all sorts of fun things like cycles and what not.