set operations should sort neighbour indices

[rsbivand]

In my rev-dep checks, nb_union failed on print because the neighbour indices were not sorted in ascending order. In print.nb, n.comp.nb was run, and chose the igraph path, so needing coercion to a sparse matrix then a graph - coercion to a sparse matrix requires indices to be given in ascending order.

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[JosiahParry]

Thank you!!