Formal set operations on query results

[RichardBruskiewich]

Some scientific problems may involve the comparison of two sets of results for intersections, or aggregation of results by set union.

Mathematical set options could be implemented in the KBA API to deal with this. If the "result caching" operations are implemented (see #59), then such set operations could be accelerated on the client side and the resulting set identified by query id (allowing for set retrieval or reuse in other queries).