Recreate simulate_root functionality from NLTK

[goodmami]

At first I hoped to avoid things like simulate_root, but I think that some people may depend on that functionality so it should probably go in.

The fake root synset will be an empty synset without an ILI, but pos is required, so it might make sense to just choose the pos of the synset needing the fake root (e.g., the one requesting the hypernym paths).