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.

Urk, tired-airport-coding. Sorry for this.  I'd actually been meaning to change the potentially silly Purpose/Substance style names to something more boring (boring is probably good), like ValueType.  Important thing here would be which is most communicative/memorable etc.