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.

We now always add a fake root node.

Having a fixed root node enables to predict the real root(s) as dependencies, i.e. using the same mechanisms as the rest of the structure.

Indeed, Li et al.'s parser has a fake root node with a unique child.