[feature] new split index implementation

[toxa81]

No description provided.

[simonpintarelli]

I've changed operator* to return an unnamed struct containing Index::global i, Index::local li, and removed the global index from the class variables. i was not updated when calling operator++, and without knowing how the class is implemented, one might use the following code:

for(auto it = begin(spl_index); it != end(spl_index); ++it) {
   (void) it.i; // Global index is accessible, but not updated
   (void) *(it.i).i; // Global index is set only in operator*
}

[simonpintarelli]

lgtm