Use `SwapDigraphs` in appropriate places

[mtorpey]

Now that #864 will imminently fix #708, we have a SwapDigraphs method that allows us to avoid messing around with !.OutNeighbours := statements and similar hacks.

We should look for places in the package where this sort of hackery happens and use SwapDigraphs instead.

[devansh2605]

These are all the instances that I found

devanshchopra@Devanshs-MacBook-Pro Digraphs % grep -R ".OutNeighbours :=" -n gap/
gap/oper.gi:1028:    D1!.OutNeighbours := nb2;
gap/oper.gi:1029:    D2!.OutNeighbours := nb1;
gap/oper.gi:1183:    D!.OutNeighbours := [];
gap/oper.gi:1214:  D!.OutNeighbours := old{list};
gap/oper.gi:1282:  D!.OutNeighbours := new;
gap/oper.gi:2257:    spanningtree!.OutNeighbours := nbs;
gap/constructors.gi:89:  D!.OutNeighbours := list;
gap/attr.gi:2438:  D!.OutNeighbours := inn;
gap/attr.gi:2541:  D!.OutNeighbours := List(DigraphVertices(D), v -> []);
gap/attr.gi:2668:      C!.OutNeighbours := out;
gap/attr.gi:2803:    C!.OutNeighbours := List([1 .. n], v -> ListBlist([1 .. n], out[v]));
gap/attr.gi:3016:  D!.OutNeighbours := DIGRAPH_SYMMETRIC_SPANNING_FOREST(D!.OutNeighbours);
gap/attr.gi:3052:  D!.OutNeighbours := OutNeighbors(C);
gap/digraph.gi:93:  record := rec(OutNeighbours := list);