Provide sense_key mapping

[goodmami]

The English Wordnet and Princeton WordNet use the dc:identifier metadata attribute on Senses to encode the sense key, but that may become obsolete in future versions of LMF (see globalwordnet/schemas#25). Since secondary resources, like sense-annotated corpora, uses sense-keys, there should be some way to look up senses by the keys.

[goodmami]

For the English WordNet, some sense keys have changed from the PWN due to spelling corrections, etc. See https://github.com/globalwordnet/english-wordnet/blob/master/src/sensekey-maps.csv