Better ordering for NNNs built with xy_nnn_hop

This ensures consistent selection of NNNs links:

> For sublattice "A": odd correspond to anti-clock-wise hoppings,
  even to clock-wise ones.

> For sublattice "B": the reverse.

src/xy_neighbors.f90

@@ -59,7 +59,7 @@ pure function xy_nnn_hop(layout,S,i) result(Ni)
       NN = xy_nn_hop(layout,S,i)
       counter = 0
       do j = 1,3
-         tmp = xy_nn_hop(layout,NN,j)
+         tmp = xy_nn_hop(layout,NN,j+i-1)
          if(tmp/=S)then
             counter = counter + 1
             NNN(counter) = tmp