It's awkward that a new clustering object needs to copy Grid cells before use

[Bjartensen]

The Clustering constructor does this

Clustering::Clustering(Grid *_g){
    g = _g;
    for (auto &v : *g->get_cells()){
        untagged_cells.push_back(v.get());
    }
}

Maybe it should only need to keep track of untagged cells by knowing the difference between tagged cells (?). I don't know which is more efficient.