BitMagic release v3.13.0

1. Implemented a new utility bm::aggregator<> for fast logic (AND, OR, AND-SUB) operations on
   groups of vectors. Aggregator uses cache blocking techniques and other optimizations to
   work faster than pairwise operations.

2. Added new example to illustrate use of new aggregator (see sample16.cpp)

3. Sparse vector search utility class (scanner) for unordered sets bm::sparse_vector<> improved its
   performance 2-3x times (via use of bm::aggregator<>).

4. Documentation update on unordered set search, new benchmark results.
   http://bitmagic.io/sparse-vector-search.html