Feature/split index proposal

[simonpintarelli]

Proposal for changes in #850:

Change operator* to:

    inline auto operator*()
    {
        return std::make_pair(idx_->global_index(this->li), this->li);
    }

And then use for (auto [i, li] : spl_index) { }. Structured bindings is a c++17 feature, but it seems to work when using c++14 standard too (adding -Wno-c++17-extensions to suppress noise)

[toxa81]

I'm working on rebase, should be ready by tomorrow

[simonpintarelli]

Unfortunately the structured bindings don't work together with openmp/clang: https://godbolt.org/z/GaM374fe4

Locally clang is even crashing when the index is used inside the openmp region, which occurs several times.