set creation / expansion improvements

[cassinius]

• [x] remove duplicates but add a counter so we have a notion of the 'strength' of elements
• [x] test input conversion
• [x] add tests for set item frequencies
• [x] also adapt accumulateSetsFromSets in graphinius-recommender accordingly
• [x] adapt similarity / recommendation scenarios based on this
• [ ] remove the Set from SetsFromSets return value => frequency map contains all we need