Add representation to store graphs with a non-dense vertex list

[MeikeWeiss]

Sometimes it is helpful to work with graphs that allow a non-dense vertex list. For instance, if I remove a vertex I would like to have still the old names. The same is the case if considering subgraphs. Of course, it is possible to get the old names by using vertex labels, but it is cumbersome. One possibility we discussed at the GAP Days would be to add a new graph representation. Then we have to make sure that some methods can be also used by this representation.