Allow specifying the points from which base-pairs are chosen, and the number of vertices of the resulting digraph objects

[wilfwilson]

Once #20 is merged, the default will be to choose all possible base-pairs in MovedPoints(G), and the digraphs will have vertices [1..LargestMovedPoint(G)] (with fixed points being isolated vertices).

• It might (?) be useful to sometimes have base-pairs that include a point that is not moved (e.g. to choose [1..4] for the group <(2,3)>?).
• It is more likely to be useful to restrict to a subset of MovedPoints(G) that G preserves (e.g. to choose [3..6] for the group <(1,2), (3,4), (5,6)>).

In addition, suppose you have the group G := <(2,3)> and want to compute its normaliser in S_4. It might be useful to have all of your orbital graphs defined on the vertices [1..4], even though you are only interested in choosing base-pairs from [2,3]. Therefore it might be useful to have a number of vertices that is different from (and in particular, greater than) LargestMovedPoint(G).