Gmap.VertexProvide a mapping function from a mapping of vertices.
val map : (G_Src.V.t -> G_Dst.vertex) -> G_Src.t -> G_Dst.tmap f g applies f to each vertex of g and so builds a new graph based on g
val filter_map : (G_Src.V.t -> G_Dst.vertex option) -> G_Src.t -> G_Dst.tfilter_map f g applies f to each vertex of g and so builds a new graph based on g; if None is returned by f the vertex is omitted in the new graph.