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.

Here you can even take advantage of Python's [generator expression syntax](https://www.python.org/dev/peps/pep-0289/).  I confess that I have this is my `$HOME/.pylintrc`:

    # I want to use map and filter; assuming others are default?
    bad-functions=apply,input

But I think you should possibly try and persuade me that I'm a very bad boy and should stop using map.  I mostly use it because I tend to like point-free style (to a limited extent)