Add LAD graph serialisation format

[james-d-mitchell]

Here's a serialiser, but no deserialiser just yet, not sure if this is correct though:

LADString := function(D)
  local result, nbs, v;
    result := StringFormatted("{}\n", DigraphNrVertices(D));
    for nbs in OutNeighbours(D) do
        Append(result, String(Length(nbs)));
        for v in SortedList(nbs) do
            Append(result, " ");
            Append(result, String(v - 1));
        od;
        Append(result, "\n");
    od;
     return result;
end;

[james-d-mitchell]

To write to a file do:

FileString(StringFormatted("filename.lad", n), LADString(D));