bxx: Added partial reduction to sugar-layer.
bxx: Added partial reduction to sugar-layer.

Example usage:

multi_array<double> x;

x = ones<double>(3,3,3);

std::cout << reduce_add(x, 0) << std::endl;

madsbk/bohrium,madsbk/bohrium,bh107/bohrium,bh107/bohrium,madsbk/bohrium,bh107/bohrium,madsbk/bohrium,bh107/bohrium