[boost] [accumulators] combine/parallel_reduce

Hi, I've just started to use the accumulators library. I must say that I like it a lot. However, I found one feature that I'm missing in the interface. 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. I'm not expert in statistical computations, but as far as I can see, this should be doable quite easily. Best Regards, Pavol.