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.

It looks here like we're exploiting the fact that RST DT numbers its EDUs from 1 and up…

*could* be worth introducing an is_fake_root to the Edu class