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.

Artificial EDUs are expected to behave well for feature extraction.
My current trial at this is to have them contain a special token, and maybe one or more special syntactic trees.
Such trees and tokens are special in that they are not real tokens or real trees.
We want to see their traces in the model (via the extracted features), but don't really want to see them in the output as they are internal to the implementation.