Added default reduction functional (TNL::Plus) to reduce

template< typename Device,

          typename Index,

          typename Fetch,

          typename Reduction >

          typename Reduction = TNL::Plus >

auto reduce( const Index begin,

             const Index end,

             Fetch&& fetch,

             Reduction&& reduction )

             Reduction&& reduction = TNL::Plus{} )

{

   using Result = std::decay_t< decltype( fetch( 0 ) ) >;

   return detail::Reduction< Device >::reduce( begin,

   return reduce< Device >( begin,

                            end,

                            std::forward< Fetch >( fetch ),

                            std::forward< Reduction >( reduction ),

 * \param reduction is a lambda function defining the reduction operation and managing the elements positions.

 * \param zero is the idempotent element for the reduction operation, i.e. element which

 *             does not change the result of the reduction.

 * \return result of the reduction in a form of std::pair< Index, Result> structure. `pair.first'

 * \return result of the reduction in a form of std::pair< Index, Result> structure. `pair.first`

 *         is the element position and `pair.second` is the reduction result.

 *

 * The `fetch` lambda function takes one argument which is index of the element to be fetched:

 * \param reduction is a lambda function defining the reduction operation and managing the elements positions.

 * \param zero is the idempotent element for the reduction operation, i.e. element which

 *             does not change the result of the reduction.

 * \return result of the reduction in a form of std::pair< Index, Result> structure. `pair.first'

 * \return result of the reduction in a form of std::pair< Index, Result> structure. `pair.first`

 *         is the element position and `pair.second` is the reduction result.

 *

 * The `fetch` lambda function takes one argument which is index of the element to be fetched:

                    Reduction&& reduction )

{

   using Result = std::decay_t< decltype( fetch( 0 ) ) >;

   return detail::Reduction< Device >::reduceWithArgument( begin,

   return reduceWithArgument< Device >( begin,

                                        end,

                                        std::forward< Fetch >( fetch ),

                                        std::forward< Reduction >( reduction ),