[feature] make some accumulators addable

Some accumulators are addable: count, sum, mean, variance.
The proposal is to support operator+ for these and for accumulator_set if all stored accumulators are addable.

Start by locating the implementations of count, sum, mean, variance, and accumulator_set in the C++ library. Review how these accumulators store and combine results, then determine the expected behavior for operator+ when all stored accumulators are addable. Done means the supported accumulators and qualifying accumulator_set instances can be added consistently.

Written by the indexing model from the issue text.