Filter queries by references...and more

[Adafede]

For now we perform all queries and then take the intersect.

It is still fast but could probably be much faster if we did not.
This would complicate a bit the code but probably be more efficient.

So instead of doing

1 = set(structures_matching_structures(wholeset)))
2 = set(structures_matching_taxa(wholeset)))
3 = set(structures_matching_references(wholeset)))

final_set = set.intersection(non_empty(1,2,3))

We could do

1 = set(structures_matching_reference(wholeset)))
2 = set(
        if(nonempty(1)){
            structures_matching_taxa(1)
            }
        else{
            structures_matching_taxa(wholeset)
            }
        )
final_set = set(
        if(nonempty(2)){
            structures_matching_structures(2)
            }
        else{
            structures_matching_structures(wholeset)
            }
        )

What do you think @bjonnh?