datasketches-cpp-4.1.0

• HLL union speed improvement
• Fixed a bug in theta and tuple union base
• new density sketch
• new count min sketch
• thread local random generator
• generic quantile sketches in Python (KLL, REQ, classic quantiles)
• generic frequent items sketch in Python
• generic tuple sketch in Python
• added optional compression of serialized theta sketch
• iterators use new style (no inheritance from std::iterator)