Use entity set as baseline instead of always defaulting to full PageRank

[Anthony Liekens]

[Anthony Liekens]

Ideally, but this may be in its own issue, users can also select "all genes" or another type of entities as their baseline.

[Anthony Liekens]

For now, I am doing experiments like this as a special case of the collection comparisons in the frontend, but having this as a feature of the computing kernel would be better.