SubSupersetGraph extension

I extended the SubSupersetGraph to contain sets and their subsets (the subsets could not be contained before).

I added a containsSubset method similar to the getExistingSubsets method but is lazy.

I added the SupersetGraph that can be used for superset requests, like the SubSupersetGraph is used for subset requests. This class uses a SubSupersetGraph internally and inverts the input and output. 

The SubSupersetGraph should be renamed to SubsetGraph, because it can only be used for subset requests. I did not rename the SubSupersetGraph, because there could be unknown references outside of the metanome repository.