Using of accumulators in paralled computation

[Pavol Droba]

In the time of multicore processors and Thread Building Blocks at hand, it would be nice to be able to use accumulators in parallel computations.

To do so, one would need to combine partial computations. In other words,we need accumulator_set::combine(). This would combine the partial computation state from another accumulator_set.

This operation would enable the accumulators to be used with parallel_reduce.