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 probably want to avoid leaving dead code lying around… even if we're intending to get to it later.  I understand though that this may be in a state of flux or fast-moving/changing stuff… just saying that dead code tends to stick around and be forgotten about.