TODO:
make constructors that accept derived stuff so we can minimize state in stats (added vals for copy)
histogram algos
- ddsketch
- frugal streaming
- T digest
- HDR
- reservoir
- linear
cardinality
- hyperloglog
- linear counting (bitset)
pgbm stuff
- vector of variance
- gradient sum (sumGrad+sumHess). this is probably just a paired sum
- crps (continuous ranked probability score)