Implement cache for GermaNet sense inventory

[logological]

Originally reported on Google Code with ID 46

The GermaNet sense inventory currently doesn't cache sense information.  This may explain
why it is extremely slow to construct a sense graph.  We should probably implement
a sense cache as we did for the WordNet sense inventories.

Reported by [email protected] on 2013-11-11 15:27:01

[logological]

Reported by [email protected] on 2013-11-12 09:43:29

• Blocked on: #47