Add a `FollowPath` operation

[james-d-mitchell]

Something like the following, where D is digraph, src is the start node of the path, and path is a list of edge indices.

FollowPath := function(D, src, path)
  local result, pos, out;

  result := [src];
  pos := 1;
  out := OutNeighbours(D);
  while pos <= Length(path) and IsBound(out[src]) and IsBound(out[src][path[pos]]) do
    src := out[src][path[pos]];
    pos := pos + 1;
    Add(result, src);
  od;
  return result;
end;