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.

I cannot replace the list comprehension with a generator expression because `writerow` expects a sequence.
Regarding `map`, I tend to forget every once in a while whether it returns a list or iterator.
List comprehensions make this explicit and save me a couple of worries when (re-)reading code.