[MISC] Layout object structure of user bins.

[smehringer]

Currently, we are indirectly requiring that the user bin ids are sequentially.
That means, if there are 5 user bins, we assume that the ids are ids = [0,1,2,3,4].

(Side question: is that fine? We think yes)

Current layout structure of layout::layout::user_bin is

    struct user_bin
    {
        std::vector<size_t> previous_TB_indices{}; // previous technical bin indices which refer to merged bin indices.
        size_t storage_TB_id{};                    // id of the technical bin that the user bin is actuallly stored in
        size_t number_of_technical_bins{};         // 1 == single bin, >1 == split_bin
        size_t idx{};                              // The index of the user bin corresponding to the order in data
    };

storing its own index.

But if we know that user bins ids are contiguous and sequential we could indirectly store the index of the user bin as its position.

E.g. user_bin.idx = position i of user bin in layout::user_bins[i]