Make WordNet's synset relations available from the lemmas

[ekaf]

Fix #1970: make WordNet's synset relations available from the lemmas.

from nltk.corpus import wordnet as wn
insect = wn.lemmas("insect")[0]
print(insect.hypernyms())

[Synset('arthropod.n.01')]

The in_topic_domain() relation ("-c") is hybrid, because a few lemmas have both lemma and synset targets. For ex:

print(insect.in_topic_domains())

[Lemma('chirpy.a.01.chirpy'), Synset('holometabolism.n.01')]

[ekaf]

This needs more work in order to return the lemmas of the synset targets, as in the last example from this comment by @marcevrard.
Alternatively, depending on the consensus, the relation definitions could be moved from the WordnetObject class to the Synset class.