pick_nth_selected(-1) should select the last node

[JEM-Mosig]

I would expect

pz.select([0, 1, 2]).at_instances_of(int).pick_nth_selected(-1)

to return the same as

pz.select([0, 1, 2]).at_instances_of(int).pick_nth_selected(2)

, but instead I get

pz.select(
  [
    0,
    1,
    2,
  ]
).at_keypaths(<0 subtrees, highlighted above>)

It'd be great if pick_nth_selected would also support negative values, as I often want to select the last item.